PNAS Formatting Requirements: Complete Author Guide

PNAS uses a standard unstructured abstract, but also requires a separate Significance Statement. Both are mandatory for Research Articles.

Abstract:

• Word limit: 250 words maximum
• Structure: Single paragraph, no subheadings
• Citations: Not allowed
• Content: Should cover the motivation, approach, key results, and implications

Significance Statement:

• Word limit: 120 words maximum
• Audience: Written for a broad scientific audience, including scientists outside your field
• Jargon: Avoid field-specific terminology. If a term wouldn't be understood by a physicist reading a biology paper, rewrite it.
• Position: Appears on the first page of the published article, below the abstract

The Significance Statement is unique to PNAS and is one of the most common sources of revision requests. Editors take it seriously because it's prominent in the published layout and is used by the media when covering PNAS papers.

A good Significance Statement explains what the problem is, why it matters, and what your paper changes. It shouldn't just restate the abstract in simpler terms. Think of it as a pitch to someone at a different department's seminar.

Bad example: "We studied the role of protein X in disease Y and found it interacts with protein Z."

Better example: "Patients with disease Y have no targeted therapies. We identify a previously unknown interaction between proteins X and Z that can be blocked by existing drugs, opening a treatment path for the 200,000 patients diagnosed annually."

PNAS doesn't enforce a strict figure cap, but figures consume page space, and the 6-page limit is rigid. In practice, most Research Articles have 4-6 figures.

Figure specifications:

PNAS-specific figure detail: Panel labels in PNAS figures are italicized uppercase letters. This is unusual. Most journals use bold, non-italic labels. If you miss this, the production team will fix it, but it signals unfamiliarity with PNAS conventions.

Table formatting:

• Tables count toward the page limit
• Every column must have a header
• Minimize horizontal rules (top, bottom, below headers only)
• No vertical rules
• Large tables should move to the SI Appendix

Color charges: PNAS used to charge extra for color figures in print. As of the current policy, color figures are published at no additional cost in both the online and print editions. This changed in recent years, so older advice about PNAS color charges is outdated.

PNAS uses a numbered citation system, but with a distinctive format that differs from most biomedical journals.

In-text citations: Numbers in parentheses (not superscript). For example: "as shown previously (1, 2)." Multiple references are separated by commas within the same parentheses. Ranges use an en dash: (3-7).

This is a common mistake. Many authors use superscript numbers when formatting for PNAS because that's standard at most biomedical journals. PNAS uses parenthetical numbers. Getting this wrong won't cause rejection, but it signals to reviewers that the formatting was done hastily.

Reference list format:

1. A. B. Smith, C. D. Jones, E. F. Williams, Title of article in sentence case. Proc. Natl. Acad. Sci. U.S.A. 120, 1234-1240 (2023).

Key formatting details:

• Author names: Initials first, then last name (e.g., "A. B. Smith")
• List all authors up to 10. For 11+, list the first 10 followed by "et al."
• Journal names use standard NLM abbreviations
• Volume number is bolded in the typeset article
• No issue numbers
• Page range followed by year in parentheses
• No DOIs in the reference list

PNAS self-citation note: When citing PNAS itself, use "Proc. Natl. Acad. Sci. U.S.A." as the abbreviation. Some reference managers default to "PNAS" which isn't the standard abbreviation for the reference list.

There's no formal reference cap, but Research Articles typically have 30-50 references. The page limit naturally constrains reference list length.

The SI Appendix is where PNAS authors put all the data that doesn't fit in the 6-page main article. It's peer-reviewed and published online alongside the paper.

SI Appendix structure:

1. SI Methods: Extended experimental procedures
2. SI Figures (Fig. S1, Fig. S2, etc.): Additional data and controls
3. SI Tables (Table S1, Table S2, etc.): Extended data tables
4. Datasets: Large datasets uploaded as separate files
5. Movies: Video files uploaded separately

SI formatting rules:

• The SI Appendix must be compiled into a single PDF for review
• It should have its own title page with the article title and "Supporting Information Appendix"
• Figures and tables are numbered with the "S" prefix (Fig. S1, Table S1)
• The SI Appendix doesn't count toward the 6-page limit
• There's no strict page cap on the SI Appendix

Practical strategy: Given PNAS's tight page limit, the SI Appendix is essential. Most successful PNAS papers have a substantial SI that contains extended methods, additional experiments, and detailed statistical analyses. The main article tells the story. The SI proves it.

Dataset deposition: PNAS requires that all data underlying the paper be available. For large datasets (genomics, proteomics, structural biology), deposition in public repositories is mandatory. Accession numbers must be cited in the Methods or SI Appendix.

PNAS has specific requirements that many other journals don't:

Author contributions statement: Mandatory. Must specify what each author did. PNAS uses its own format: "Author contributions: A.B.S. designed research; C.D.J. performed research; E.F.W. analyzed data; and A.B.S. wrote the paper." Use author initials, not full names.

Article classification: At submission, you must select a PNAS classification for your article:

• Physical Sciences (with subcategories like Applied Mathematics, Chemistry, Physics)
• Biological Sciences (with subcategories like Biochemistry, Cell Biology, Neuroscience)
• Social Sciences (with subcategories like Economics, Psychology, Political Sciences)

This classification determines which editor and reviewers handle your paper. Choose carefully. A paper classified as "Immunology" will reach different reviewers than one classified as "Cell Biology," even if the content overlaps.

Article type designation: Research Articles can be designated as either "Direct Submission" or "Contributed." Contributed articles are submitted by NAS members and follow a different review process. Most authors submit through the Direct Submission track.

ORCID is mandatory for the corresponding author. Won't be processed without it.

Title length. PNAS prefers short, specific titles. There's no strict character limit, but the editorial style favors brevity. Avoid subtitles with colons when possible.

No separate Introduction heading. Like Nature and Science, PNAS papers don't use a formal "Introduction" heading. The opening section is untitled. The first heading in the body is typically "Results" or a Results subheading.

Results before Discussion. PNAS uses the order: untitled introduction, Results, Discussion, Methods. Methods come last in the main text, before references. Some authors place Materials and Methods before Results, and PNAS doesn't reject this, but the Results-first convention is standard.

Footnotes for affiliations. PNAS uses footnotes (not endnotes) for author affiliations, designated with superscript lowercase letters. This is handled by the template but worth knowing.

Abbreviations. Define at first use in the abstract and again in the body text. Standard abbreviations (DNA, RNA, ATP) don't need definition.

Classification codes. PNAS assigns subject classification codes to every article. Authors suggest codes at submission, but editors make the final assignment.

Competing interest statement. Mandatory. Use the exact phrasing "The authors declare no competing interest" if none exist.

1. Significance Statement is separate from the abstract. Don't combine them. They serve different purposes and appear in different locations on the published page.

1. Page count includes everything. Title, author list, abstract, Significance Statement, body text, references, and figure legends all count toward the 6 pages. Only the SI Appendix is excluded.

1. Line numbers. Required on all pages of the manuscript for review.

1. Double spacing. Required for the submission manuscript.

1. Footnotes vs endnotes. PNAS uses footnotes, not endnotes. This affects how you set up your Word or LaTeX document.

1. Figure legends go at the end of the manuscript. After the reference list, not under the figures.

1. Ethics statements. IRB and IACUC approvals must be stated in the Methods section with protocol numbers.