A short peptide with an unusually long research history, most of it published outside the English-language literature.
Epitalon
Epitalon is a synthetic tetrapeptide, alanyl-glutamyl-aspartyl-glycine, developed as a synthetic counterpart to Epithalamin, a pineal gland extract studied in the Soviet Union from the 1970s onward. Four amino acids is about as small as a peptide gets while remaining a peptide.
The programme behind it is unusual. Rather than emerging from a pharmaceutical pipeline, Epitalon came out of a sustained state-backed research effort into pineal regulation and ageing, led principally by Khavinson at the St Petersburg Institute of Bioregulation and Gerontology. That lineage is why much of the literature is Russian-language and less familiar to Western readers.
Published work covers rodent lifespan studies, effects on melatonin rhythm, and cell culture models examining telomerase activity. Reviews consolidating the programme have appeared in English-language journals. Readers should note that much of the primary work predates modern reporting standards, and independent replication outside the original groups is limited.