Thymulin is a nonapeptide hormone of thymic epithelial cell origin, sequence pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn, with an N-terminal pyroglutamate and a free C-terminal carboxyl. PubChem CID 71300623 records the metal-free peptide as C33H54N12O15, 858.9 g/mol, under CAS 78922-62-0, and its IUPAC name independently confirms the residue order. Unlike the other entries in this group it is not a Khavinson bioregulator and has an entirely separate provenance: it was characterised in France by Bach and Dardenne as facteur thymique serique, and "FTS" in the older literature denotes the zinc-free apopeptide (see PMID 2189748, which describes FTS explicitly as "zinc-free thymulin"). The distinction is chemical, not semantic — thymulin proper is the zinc complex.

Zinc coordination is the defining structural feature. The 2004 Peptides review "Thymulin and the neuroendocrine system" describes it as a thymic hormone exclusively produced by thymic epithelial cells, consisting of a nonapeptide component coupled to the zinc ion, which confers biological activity (PMID 15003367). Study systems in the indexed literature include T-cell differentiation assays, hypothalamic-pituitary-thymic axis work, gonadotropin-releasing activity in pituitary cell preparations across age (PMID 10811143), and endotoxin-stimulated alveolar epithelial cells, where thymulin and zinc were examined against proinflammatory cytokine production, cAMP-dependent signalling and NF-kB nuclear translocation (PMID 19850345).

On the evidence base: thymulin differs from the rest of this group in that it is a mainstream immunology subject with a large multi-national literature going back to the 1970s, including substantial independent replication of its existence, zinc dependence and thymic origin. What it does not have is a therapeutic track record. No thymulin product holds a marketing authorisation in any jurisdiction I could verify, contemporary work remains preclinical, and analytical handling is non-trivial because the zinc-free and zinc-bound forms are distinct species with different activity. Material supplied as "thymulin" is in practice usually the synthetic apopeptide.