RADER, Circuit Judge.
In an interference over a new assay to identify anti-cancer compounds, the United States Patent and Trademark Office Board of Patent Appeals and Interferences (Board) awarded priority to Mariano Bar-bacid and Veeraswamy Manne (collectively Barbacid) over Michael Brown, Joseph Goldstein, and Yuval Reiss (collectively Brown). Because the Board did not consider evidence that Brown conceived the invention before Barbacid reduced it to practice and diligently pursued the invention from the time of Barbacid’s reduction to practice through Brown’s filing date, this court vacates the award of priority to Barbacid and remands.
BACKGROUND
This case involves an interference between U.S. Patent No. 5,185,248 (the Bar-[1331]*1331bacid patent) and U.S. patent application Serial No. 07/937,893 (the Brown application). The Barbacid patent and the Brown application both claim an assay for identifying new anti-cancer compounds that inhibit farnesyl transferase (FT), an enzyme involved in the control of cell growth. FT functions in the cell by adding farnesyl (a branched-chain polyunsaturated hydrocarbon alcohol intermediate of sterol biosyn-thesis) to a cysteine amino acid near one end of the protein chain, namely the car-boxy-terminus. An important protein susceptible to addition of farnesyl is “ras.” The farnesylation reaction activates the ras protein (which stimulates cell growth) by moving ras to the vicinity of the cell membrane. Once near the membrane, ras stimulates cell growth. Thus, ah FT inhibitor would reduce the amount of ras reaching the membrane and therefore reduce ras-stimulated growth (including “cancerous” growth).
The sole count in the interference provides:
A method for identifying a candidate substrate having the ability to inhibit a farnesyl transferase enzyme, comprising the steps of:
6. obtaining an enzyme composition comprising a farnesyl transferase enzyme that is capable of transferring a farnesyl moiety to a farnesyl acceptor substance;
7. admixing a candidate substrate with the enzyme composition and fame-syl pyrophosphate; and
8. determining the ability of the farne-syl transferase enzyme to transfer a farnesyl moiety to a farnesyl acceptor substrate in the presense of the candidate substance and in the ab-sense of the candidate substance.
OR
An assay for identifying compounds that inhibit ras oncogene activity, comprising:
6. reacting a protein or peptide substrate having a CAAX motif with farnesyl pyrophosphate and fame-syl-protein transferase in the presence of a test substrate, and
7. detecting wither the farnesyl residue is incorporated into the protein or peptide substrate, in which the ability of the test substance to inhibit ras oncogene activity is indicated by a decrease in the incorporation of the farnesyl residue into the protein or peptide substrate as compared to the amount of the farnesyl residue incorporated into the protein or peptide substrate in the absence of the test substrate.
Barbacid & Manne v. Brown, Goldstein & Reiss, Interference No. 103,586, slip op. at 2-3 (Bd. Pat. Appeals & Interferences May 30, 2000) {Board opinion) (emphasis added). In other words, the method of the count uses: (1) farnesyl transferase (FT); (2) farnesyl pyrophosphate, i.e., the source of farnesyl; (3) a “farnesyl acceptor substance” or “protein or peptide substrate having a CAAX motif,” i.e., ras or a peptide of ras containing the CAAX motif (which is farnesylated); and (4) a test or candidate substrate, which inhibits FT, and therefore, ras protein activity.
The Barbacid patent application was filed on May 8, 1990, and issued on February 9, 1993. The Brown application was filed on December 22, 1992, but was accorded the benefit of an earlier related application filed on April 18, 1990. Thus, Brown was the senior party. Barbacid, as the junior party, had the burden to prove [1332]*1332priority by a preponderance of the evidence.
The Board found that Barbacid showed an actual reduction to practice no later than March 6, 1990. The Board also found that Brown did not show reduction to practice of the count before March 6, 1990. Specifically, the Board found that Dr. Yu-val Reiss’ September 20, 1989 FT experiment did not satisfy every limitation of the count because it did not include a test or candidate substance in the assay. The Board also discounted a September 25, 1989 experiment (which may have satisfied the count) because Dr. Reiss could not authenticate his lab notebooks and autora-diographs. Moreover Dr. Patrick Casey could not corroborate Dr. Reiss’ testimony and documents relating to the September 25 experiment.
Responding to a request for reconsideration, the Board declined to consider the testimony of Debra Morgan with respect to Brown’s earlier conception and reduction to practice. The Board found that Brown’s sole reference to Ms. Morgan in their opening brief was in a section of the Statement of Facts entitled “Brown’s Case for Diligence and Corroboration Thereof,” but not in the Argument section. Thus, the Board denied Brown’s request for reconsideration of its holding that Brown had not reduced the invention to practice before March 6, 1990. Accordingly, the Board awarded priority to Barbacid. Brown appealed.
DISCUSSION
Priority and its issues of conception and reduction to practice are questions of law predicated on subsidiary factual findings. Cooper v. Goldfarb, 154 F.3d 1321, 1327, 47 USPQ2d 1896, 1901 (Fed.Cir.1998). Accordingly, this court reviews without deference the Board’s legal conclusions on priority, conception, and reduction to practice, Hybritech Inc. v. Monoclonal Antibodies, Inc., 802 F.2d 1367, 1376, 231 USPQ 81, 87 (Fed.Cir.1986), and reviews for substantial evidence the Board’s factual findings. Dickinson v. Zurko, 527 U.S. 150, 119 S.Ct. 1816, 144 L.Ed.2d 143 (1999); In re Gartside, 203 F.3d 1305, 1315, 53 USPQ2d 1769, 1775 (Fed.Cir.2000). Finally this court reviews the Board’s application of its permissive interference rules for an abuse of discretion. Abrutyn v. Giovanniello, 15 F.3d 1048, 1050, 29 USPQ2d 1615, 1617 (Fed.Cir.1994) (citing Gerritsen v. Shirai, 979 F.2d 1524, 1527-28, 24 USPQ2d 1912, 1915-16 (Fed.Cir.1992)).
I.
In an interference with an application filed after the date of the patent, the junior party must show priority by clear and convincing evidence. 37 C.F.R. § 1.657(c) (2001); Price v. Symsek, 988 F.2d 1187, 1190-91, 26 USPQ2d 1031, 1033 (Fed.Cir.1993). In interferences, such as this case, with an application whose effective filing date antedates the patent issuance, the junior party must show priority by a preponderance of the evidence. 37 C.F.R. § 1.657(c); Bosies v. Benedict, 27 F.3d 539, 541-42, 30 USPQ2d 1862, 1864 (Fed.Cir.1994). Barbacid, as the junior party, has the ultimate burden to prove priority. Id. Brown asserts that the Board inappropriately shifted the burden of proof by requiring Brown to show by a preponderance conception or reduction to practice before March 6, 1990 — the date of Barbacid’s actual reduction to practice. Brown argues that the Board should have shifted to Brown the burden of production — the [1333]*1333burden of going forward with sufficient evidence — rather than the burden of proof.
This court has not addressed whether a senior party has the burden to show by a preponderance a date of invention before the priority date shown by the junior party. The Board cites to a Board decision, Fisher v. Gardiner, 215 USPQ 620, 625, 1981 WL 48136 (Bd. Pat. Interferences Oct. 30, 1981) (“Inasmuch as Fisher et al. [the junior party] have established a reduction to practice of the subject matter in counts 1, 2 and 4 prior to the senior party’s filing date, the burden shifts to Aymami [the senior party] to prove by a preponderance of the evidence a priority date for that subject matter earlier than the July 12,1973 date established by Fisher et al.”). Board opinion, slip op. at 12.
To the contrary, 37 C.F.R. § 1.657(a) states: “A rebuttable presumption shall exist that, as to each count, the inventors made their invention in the chronological order of their effective filing dates. The burden of proof shall be upon a party who contends otherwise ” (emphasis added). Paragraph (b) of the same section explains that the junior party has the burden of establishing priority by a preponderance of evidence. 37 C.F.R. § 1.657(b). In other words, the burden of proof by a preponderance of the evidence “shall be on a party” contending they made their invention out of chronological order of the effective filing dates, i.e., the junior party. This burden of proof does not shift.
Irrespective of that burden, however, both parties must be given an opportunity to submit evidence regarding priority in an interference proceeding. Once all evidence has been submitted, the Board must assess, in light of all the evidence presented by both parties, whether the junior party has met its ultimate burden of proving priority by preponderance of the evidence.
In sum, under 37 C.F.R. § 1.657(a) and (b), the ultimate burden of proof always remained on the junior party, Barbacid. Thus, the Board erred in stating that the burden of proof shifted to Brown at any point in this case. Notwithstanding that error, this court must still determine whether the record supports the Board’s award of priority to Barbacid. Specifically, this court (or the Board on remand, as the case may be) must determine, based on the entire evidentiary record, whether Barbacid ultimately prevailed in proving priority by a preponderance of evidence.
II.
Brown alleges that the Board erred in denying authentication to Dr. Reiss’ lab notebooks and autoradiographs under 37 C.F.R. § 1.671(f). Paragraph (f) of § 1.671 (entitled “Evidence must comply with rules”) states: “The significance of documentary and other exhibits identified by a witness in an affidavit or during oral deposition shall be discussed with particularity by a witness.” 37 C.F.R. § 1.671(f) (2001) (emphasis added). The Board noted that § 1.671(f) requires a witness to explain the entries of various pages of the lab notebooks and exhibits. Cf. Fed. R.Evid. 902 (excluding notes and lab notebooks from the list of self-authenticating extrinsic evidence). The Board found that Dr. Reiss did not give sufficient testimony regarding specific entries in his lab notebook or on relevant autoradiographs (i.e., Exhibit 32). Without an adequate explanation of Exhibit 32, the Board rejected the exhibit for lack of authentication.
Exhibit 32 refers to notebook pages and autoradiographs from Dr. Reiss’ experi-[1334]*1334merits from August to October 1989, including experiments dated September 20 and September 25, 1989. With regard to the September 25 experiment, Dr. Reiss stated in paragraph 24 of his declaration:
On September 25, 1989, I conducted an assay to determine the pH dependence of the farnesyl transferase preparation currently under use (Exhibit 32; pages 0035 to 0039). This study employed a peptide considered to be a potential inhibitor of ras farnesylation. This peptide comprised the carboxy-terminus ten amino acids of the ras molecule. The format of this assay was the gel electrophoresis format, described above in paragraph 20 [discussing the September 20 experiment]. The radioautograph developed from the corresponding gel (Exhibit 32; page 0038) clearly shows that inclusion of peptide at 10 and 20 ig (lanes 14 and 15, respectively) inhibited farnesyl transferase-mediated labeling of ras by 14C-FPP, as determined by the reduction/absence of ras-speeific bands in these lanes.
This explanation informs one of skill in the art, upon a review of the relevant autoradiographs and lab notebook pages in Exhibit 32, that Dr. Reiss conducted an FT experiment on September 20, 1989, and then conducted another FT assay using a peptide inhibitor on September 25, 1989. Moreover, an examination of the September 25 autoradiograph from those experiments, specifically lanes 14 and 15 (which can be identified by counting lanes starting from the left), shows that farnesyl transferase-mediated labeling of ras by14 C-FPP was reduced in the presence of the inhibiting peptide.
Dr. Reiss did not analyze every lane in the autoradiograph. For example, he did not expressly state which bands in the gels corresponded to the labeled ras protein. Nor did Dr. Reiss discuss the molecular weight markers (in lane 1 on the left of the autoradiograph). Likewise, he did not describe each experiment in every single lane of the gels. Nevertheless, comparing lanes 2-11 to lanes 14-15 in the September 25 autoradiograph, one of skill in this art would understand that Dr. Reiss had inhibited ras farnesylation in the presence of the peptide.
While Dr. Reiss could have discussed the September 25 experiment in more detail, the Board must nonetheless weigh that evidence from the vantage point of one of skill in the art. See Mahurkar v. C.R. Bard, Inc., 79 F.3d 1572, 1578, 38 USPQ2d 1288, 1292 (Fed.Cir.1996) (stating that the trier of fact can conclude for itself what documents show, aided by testimony about the meaning of the exhibit to one skilled in the art). In this case, the notebook data itself explains the methods and results of the September assays. Thus, in light of Dr. Reiss’ testimony, one of skill in this art would understand Exhibit 32 relating to the September experiments.
In excluding Exhibit 32 for lack of authentication, the Board applied its own rule. This court reviews the Board’s application of its rules for an abuse of discretion. Abrutyn, 15 F.3d at 1050 (citing Gerritsen v. Shirai, 979 F.2d at 1527-28 (Fed.Cir.1992)). Notwithstanding that high standard of review, this court finds that the Board abused its discretion by excluding evidence within the understanding of skilled artisans when considering authentication requirements. See Mahurkar, 79 F.3d at 1578.
III.
Brown further argues that the Board erred in refusing to allow an inven[1335]*1335tor’s own documentation to corroborate his conception or reduction to practice. A party seeking to prove conception via the oral testimony of a putative inventor must proffer evidence corroborating that testimony. Singh v. Brake, 222 F.3d 1362, 1367 (Fed.Cir.2000); Mahurkar, 79 F.3d at 1577; Price v. Symsek, 988 F.2d 1187, 1194, 26 USPQ2d 1031, 1036 (Fed.Cir.1993). This corroboration rule does not apply with the same force to proof of inventive facts with physical exhibits. Ma-hurkar, 79 F.3d at 1577-78 (“This court does not require corroboration where a party seeks to prove conception through the use of physical exhibits. The trier of fact can conclude for itself what documents show, aided by testimony as to what the exhibit would mean to one skilled in the art.”); Price, 988 F.2d at 1195-96; Loom Co. v. Higgins, 105 U.S. 580, 594, 26 L.Ed. 1177 (1882).
Thus, Brown’s physical evidence, such as Dr. Reiss’ notebooks and autoradiographs, do not require corroboration to demonstrate the content of the physical evidence itself, namely that FT assay experiments took place on September 20 and 25, 1989. Conversely, however, the physical evidence in this case may not singlehandedly corroborate Dr. Reiss’ testimony. See Price, 988 F.2d at 1195 (“Unlike a situation where an inventor is proffering oral testimony attempting to remember specifically what was conceived and when it was conceived ... ‘corroboration’ is not necessary to establish what a physical exhibit before the board includes. Only the inventor’s testimony requires corroboration before it can be considered.”); Hahn v. Wong, 892 F.2d 1028, 1032, 13 USPQ2d 1313, 1317 (Fed.Cir.1989); Blicharz v. Hays, 496 F.2d 603, 605-06 (CCPA 1977). Thus, an inventor’s testimonial assertions of inventive facts require corroboration by independent evidence. Thomson S.A. v. Quixote Corp., 166 F.3d 1172, 1174-75, 49 USPQ2d 1530, 1533 (Fed.Cir.1999), cert. denied, 527 U.S. 1036, 119 S.Ct. 2395, 144 L.Ed.2d 796 (1999); Cooper v. Goldfarb, 154 F.3d 1321, 1330, 47 USPQ2d 1896, 1903 (Fed.Cir.1998).
This court applies a “rule of reason” analysis to determine sufficient corroboration. Singh, 222 F.3d at 1367; Price, 988 F.2d at 1195. In applying the “rule of reason” test, this court examines “all pertinent evidence” to determine the credibility of the “inventor’s story.” Price, 988 F.2d at 1195. This “rule of reason” analysis does not alter the requirement of corroboration for an inventor’s testimony. The inventive facts must not rest alone on testimonial evidence from the inventor himself. Cooper, 154 F.3d at 1330. As stated in Hahn, 892 F.2d at 1032, “[t]he inventor ... must provide independent corroborating evidence in addition to his own statements and documents.” See also Knorr v. Pearson, 671 F.2d 1368, 1373, 213 USPQ 196, 200 (CCPA 1982) (“[S]ufficient circumstantial evidence of an independent nature can satisfy the corroboration rule.”).
Thus, independent evidence must corroborate Dr. Reiss’ testimony of conception or actual reduction to practice. The Board did not err in holding that an inventor’s own unwitnessed documentation does not corroborate an inventor’s testimony about inventive facts.
IV.
Conception is “the formation in the mind of the inventor[ ] of a definite and permanent idea of the complete and operative invention, as it is thereafter to [1336]*1336be applied in practice.” Singh, 222 F.3d at 1367 (quoting Kridl v. McCormick, 105 F.3d 1446, 1449, 41 USPQ2d 1686, 1689 (Fed.Cir.1997) (internal quotation marks omitted)). A conception must encompass all limitations of the claimed invention, see id., and “is complete only when the idea is so clearly defined in the inventor’s mind that only ordinary skill would be necessary to reduce the invention to practice, without extensive research or experimentation,” Id. (quoting Burroughs Wellcome Co. v. Barr Lab., Inc., 40 F.3d 1223, 1228, 32 USPQ2d 1915, 1919 (Fed.Cir.1994)).
As correctly found by the Board, Dr. Reiss did not satisfy every limitation of the count when he conducted his FT assay experiment on September 20, 1989. The laboratory notebook and autoradiograph themselves show that the September 20 experiment did not include the use of a test/candidate substrate (i.e., an inhibitor of FT) — an element of the count. Likewise, in the only independent testimony corroborating Dr. Reiss’ experiments, Dr. Casey did not suggest that the September 20 experiment included an FT inhibitor. Thus, the physical and testimonial evidence regarding the September 20 experiment do not show conception or reduction to practice.
V.
Unlike the September 20 experiment, the September 25 experiment included a peptide inhibitor of FT in the FT assay. Thus, the September 25 experiment contained all of the limitations of the count. As discussed above, however, independent evidence (testimony or physical evidence from a source other than Dr. Reiss) must corroborate Dr. Reiss’ testimony to show an actual reduction to practice. In other words, Dr. Casey’s testimony, the only other relevant independent evidence available, must corroborate Dr. Reiss’ own statements and documents to show a reduction to practice on September 25, 1989. Cooper, 154 F.3d at 1330. Dr. Casey’s testimony could not corroborate Dr. Reiss’ testimony regarding the September 25 experiment, however, because Dr. Casey did not purport to witness the September 25 autoradiograph. Nor did Dr. Casey purport to discuss the September 25 experiment in particular with Dr. Reiss at any time.
In his declaration submitted to the Board, Dr. Casey stated:
8. On Thursday, September 14, 1989, Dr. Janice Buss came to Southwestern Medical School to present a seminar. I recall that within a week or so of that date, Dr. Reiss showed me the results of a study in which he had demonstrated farnesyl transferase activity in a gel-based assay.... [Description of the experiment] I distinctly recall this study, as it was a very important showing. The notebook page shown in Exhibit 32 as page 0031 [dated September 20, 1989] is the experiment Dr.' Reiss showed to me....
9. In the latter part of September, 1989, there was a major development in my own research project that consumed my efforts, and distracted me from the farnesyl transferase project, for about one month. I recall, however, that by at least about the end of October or the beginning of November, I was aware that Dr. Reiss had demonstrated that short peptides, derived from ras, inhibited farnesyl transferase in vitro in the gel-based assay described above.
Thus, Dr. Casey did not discuss the September 25 experiment in his declaration. Consequently, the Board did not err when [1337]*1337it determined that evidence regarding the September 25, 1989 experiment did not show a reduction to practice.
On the other hand, the physical evidence itself — the September 25 lab notebook pages and autoradiographs — show that an experiment containing all elements of the count took place on that date. As discussed above, this physical evidence requires no further corroboration to demonstrate the content of the physical evidence itself. Mahurkar, 79 F.3d at 1577; Price, 988 F.2d at 1195-96. In addition, while Dr. Casey’s vague testimony does not corroborate Dr. Reiss’ testimony of an actual reduction to practice, Dr. Casey’s testimony certainly suggests that Dr. Reiss had the idea of combining the FT assay with the use of FT peptide inhibitors sometime before the end of October or the beginning of November 1989. Thus, Dr. Casey’s independent testimony corroborates Dr. Reiss’ testimony of a conception before November 1989.
In the Facts section of their brief to the Board, Brown stated that they conceived of the invention by September 25, 1989, when that assay showed both FT activity and an inhibition of FT activity by candidate inhibitors. Brown Opening Brief at 13. Moreover, in their Argument section, under “Brown’s First Alternative Case for Priority — ‘Simultaneous’ Conception and Reduction to Practice,” Brown argued (albeit in the alternative and primarily in the section title itself) that Brown both conceived and reduced to practice their invention on September 25, 1989. Id. at 33. Brown also cited Dr. Reiss’ September 25 lab notebook pages and autoradiographs, as well as Dr. Casey’s independent corroboration of Dr. Reiss’ testimony regarding conception before the end of October or the beginning of November 1989. Id.
Despite Brown’s argument and citation to relevant physical and testimonial evidence, the Board did not address whether the September 25 experiment demonstrated conception. Board opinion, slip op. at 17-22. The Board only addressed whether the September 20 experiment demonstrated conception and whether the September 25 experiment demonstrated an actual reduction to practice. Moreover, the Board noted: “Without a conception, the issue of reasonable diligence by the inventors to a reduction to practice is moot. Accordingly, we have not considered any evidence relating to diligence.” Id., slip op. at 20, n. 15.
Priority of invention “goes to the first party to reduce an invention to practice unless the other party can show that it was the first to conceive the invention and that it exercised reasonable diligence in later reducing that invention to practice.” Price, 988 F.2d at 1190. Because Brown asserted to the Board conception of the invention on September 25, and invoked physical evidence that did not require corroboration, as well as testimony by Dr. Casey corroborating Dr. Reiss’ testimony regarding conception, the Board erred in failing to consider whether the September 25 lab notebook pages and autoradiographs themselves, especially in light of the independent testimony by Dr. Casey, demonstrated conception by’Brown. Likewise, the Board erred in failing to consider whether Brown was diligent from March 6, 1990, the date of Barbacid’s actual reduction to practice, until Brown’s filing date on April 18, 1990. See Brown Opening Brief at 34.
VI.
The Board found that Brown’s sole reference to Ms. Morgan in their [1338]*1338Opening Brief was found in their section of the Facts entitled “Brown’s Case for Diligence and Corroboration Thereof’ (emphasis added). The Board also found that the Argument section of Brown’s brief did not rely on Ms. Morgan’s testimony at all. Thus, based on 37 C.F.R. § 1.656(b), the Board declined to consider the testimony of Ms. Morgan with respect to Brown’s conception and reduction to practice.
37 C.F.R. §§ 1.656(b) and (c) state that the opening brief of a junior and senior party shall contain, inter alia (emphasis added):
(5) A statement of the facts, in numbered paragraphs, relevant to the issues presented for decision with appropriate references to the record.
(6) An argument, which may be preceded by a summary, which shall contain the contentions of the party with respect to the issues it is raising for consideration at final hearing, and the reasons therefor, with citations to the cases, statutes, other authorities, and parts of the record relied on.
37 C.F.R. § 1.656(b), (c) (2001).
In their Opening Brief, Brown stated in the Facts section, under “D. Brown’s Case for Diligence and Corroboration Thereof’, in paragraph 24, that Ms. Morgan, a laboratory technician in Dr. Goldstein’s and Brown’s research group, conducted FT assays using various candidate inhibitors on February 27 and 28, as well as a number of days in March and April 1990. In their Argument section, Brown referenced Ms. Morgan’s work only to say that: “Extensive evidence of diligence can be found in AX32, AX33, AX34, AX35, AX36, AX39, AX40, AX41 and AX46.” In other words, Brown referred to Ms. Morgan’s FT assays using candidate inhibitors only with regard to their argument of reasonable diligence by Brown between their conception date and Brown’s filing date of April 18,1990.
Thus, the Board did not abuse its discretion when it declined to consider Ms. ■ Morgan’s testimony with respect to Brown’s conception and reduction to practice. For the reasons discussed above, however, the Board did abuse its discretion in refusing to consider Ms. Morgan’s testimony with regard to reasonable diligence by Brown from March 6, 1999, until the filing of their patent application on April 18,1990.
CONCLUSION
Because the Board did not consider the September 25, 1989 experiment or Dr. Casey’s corroborating testimony with regard to conception by Brown, or any evidence of reasonable diligence by Brown between the date of Barbacid’s actual reduction to practice and the filing of Brown’s patent application on April 18, 1990 (such as Ms. Morgan’s testimony discussed above), this court vacates the award of priority to Bar-bacid. Accordingly, this court remands this case back to the Board for further proceedings on Brown’s conception and reasonable diligence.
VACATED and REMANDED.