How big is the human proteome?

Humans do not have one proteome size. The shortest useful answer is about 19,400 proteins: one reference protein for each protein-coding gene.

That is the parts-list answer, not the full molecular answer. Current annotation already contains 172,117 distinct translated products, and the number of proteoforms, the actual molecular forms proteins take in cells, is likely in the millions.

How large is the human proteome?

The human proteome is about 19,400 proteins when counted as one reference protein per protein-coding gene. GENCODE Release 50 lists 19,442 protein-coding genes, while the 2025 HUPO Human Proteome Project reference proteome contains 19,435 proteins.^[1][2]

These two numbers are close because they are both trying to count the same narrow object: the human protein parts list. They are not identical because reference datasets use different releases, evidence rules, and curation workflows.

The Human Proteome Project reported that 93.6 percent of its 19,435-protein reference proteome had been detected with protein-level evidence in its 2025 report.^[2] In plain terms, the narrow human proteome is now mostly observed, but not fully closed.

The table uses GENCODE Release 50 for annotation counts and the 2025 HUPO HPP report for reference-proteome detection.^[1][2]

Why is the proteome larger than the genome?

The proteome is larger than the genome in product count, not in DNA length. A human has about 19,400 protein-coding genes, but those genes can produce many transcript isoforms, translated products, and chemically modified protein forms.

GENCODE v50 lists 278,455 protein-coding transcripts and 172,117 distinct translations from 19,442 protein-coding genes. It also reports that 16,058 genes have more than one distinct translation.^[1]

This is why a human gene count is not the same thing as a human protein count. The genome stores the instructions. The proteome is the changing set of protein products made from those instructions in particular cells, tissues, developmental stages, and disease states.

The difference becomes larger after translation. Proteins can be cleaved, phosphorylated, acetylated, methylated, glycosylated, oxidized, or combined with sequence variants. Those molecular forms can behave differently even when they come from the same gene.

How many human proteoforms are there?

There is no fixed count of human proteoforms. The best defensible answer is that the number is likely in the millions, but it depends on the cell type, abundance threshold, measurement method, and definition of a distinct molecular form.

A 2018 review framed the scale clearly: one representative product per human gene gives a proteome of about 20,000; splice-level counting raises the number to around 70,000; immune receptor recombination can create billions of potential variants in some cell types. For a practical cell-type target, the authors used 10,000 expressed genes times 100 proteoforms per gene, or about 1 million proteoforms in a given cell type.^[3]

The earlier "width and depth" framing is useful here. The width of the proteome is the number of different protein species or proteoforms; the depth is how many copies of those proteins are present in a tissue or cell state. A 2016 estimate based on protein-database trends placed the human proteoform scale around 6 million, but that should be read as an estimate, not a census.^[4][3]

The boundary is still moving. In 2026, the TransCODE consortium analyzed 7,264 non-canonical open reading frames and found peptide evidence for about a quarter of them across large-scale proteomics datasets. The study introduced "peptideins" for experimentally detected translation products whose conventional protein-coding status is still unsettled.^[5]

That matters because the human proteome is not just a list to finish. It is a measurement problem: which protein forms exist, in which cells, at what abundance, and under which conditions.

What is the Human Proteome Project?

The Human Proteome Project is HUPO's international effort to turn the predicted human protein parts list into an experimentally supported map of human proteins and their functions.

Its 2025 report describes a 19,435-protein reference proteome based on GENCODE v48, UniProtKB 2025_03, Human Protein Atlas 24, MassIVE-KB 2023, and PeptideAtlas 2025-01. The project reported 93.6 percent protein detection and 5,562 proteins in the highest function-evidence category.^[2]

This makes the project close to complete for the narrow question: "Has at least one protein product from each reference protein-coding gene been detected?" It is much further from complete for the deeper question: "Do we know every proteoform, abundance pattern, interaction, location, and function across human biology?"

That is the most honest answer to human proteome size. The reference proteome is about 19,400 proteins. The annotated translated-product set is about 172,000. The proteoform-resolved human proteome is likely measured in millions and is still being mapped.

Sources▼

1. Human release statistics (version 50) GENCODE · July 1, 2026. https://www.gencodegenes.org/human/stats.html
2. The 2025 Report on the Human Proteome from the HUPO Human Proteome Project Journal of Proteome Research · 2026. https://doi.org/10.1021/acs.jproteome.5c00759
3. How many human proteoforms are there? Nature Chemical Biology · 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC5837046/
4. The Size of the Human Proteome: The Width and Depth International Journal of Analytical Chemistry · 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC4889822/
5. Expanding the human proteome with microproteins and peptideins Nature · 2026. https://www.nature.com/articles/s41586-026-10459-x