Peptideins are a newly named class of molecules found within human cells, similar to proteins but smaller and with vaguer purposes. They are part of the "dark proteome" of microproteins, long dismissed as too small to matter. The name was coined in a paper in Nature in May 2026 co-led by Sebastiaan van Heesch of the Princess Máxima Centre in the Netherlands. They may unlock new drug targets, including for immunotherapies and vaccines against cancer.
Cells make proteins by copying DNA into RNA, then translating RNA into chains of amino acids known as peptides; proteins are a loosely defined subset of peptides that are of a certain size, with known functions found across species. There are about 19,500 recognised human proteins. By pooling and analysing previous experiments, van Heesch and colleagues confirmed the existence of 1,785 microproteins—about 65% of them fewer than 50 amino acids long, smaller than 99% of known proteins. Some, such as ASNSD1-uORF (involved in medulloblastoma) and humanin (which protects cells from stress and may play a role in healthy ageing), are already known to be biologically useful.
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