Testagen is a synthetic tetrapeptide and Khavinson-class peptide bioregulator derived from testicular tissue, investigated for regulatory effects on testicular cell function and age-related reproductive hormone decline. Like other Khavinson-class bioregulators, testagen is proposed to act as a short regulatory peptide that modulates gene expression in target organ cells — in this case, testicular Leydig cells responsible for testosterone synthesis and Sertoli cells that support spermatogenesis. The Khavinson bioregulator model posits that short peptides derived from specific tissues preferentially interact with chromatin regulatory elements in homologous tissues, restoring expression of age-downregulated genes and partially reversing organ-level functional decline associated with aging. Research interest in testagen centers on age-associated decline in testicular function, including reduced Leydig cell activity and the associated decline in testosterone biosynthesis that characterizes male aging. Preclinical studies have examined testagen-class peptides for effects on reproductive hormone levels, testicular histomorphology, and markers of gonadal function in aged animal models. Testagen is part of the broader Khavinson bioregulator research program that has also characterized thymalin (thymus), cortagen (cardiovascular/neural), cartalax (cartilage), and pinealon (pineal gland) as organ-specific regulatory peptides. Testagen benefits investigated in preclinical research include support for testosterone biosynthesis pathways, maintenance of Leydig cell populations, and preservation of testicular tissue architecture with aging. These effects are proposed to occur through epigenetic regulatory mechanisms rather than direct hormonal supplementation, distinguishing bioregulator peptides from exogenous testosterone replacement. Testagen is classified as a research compound with no regulatory approval in any jurisdiction. No human clinical trials have established safety or efficacy for testicular support, testosterone augmentation, or reproductive function endpoints. Research protocols have examined testagen primarily via subcutaneous injection, with oral delivery also studied in bioregulator research contexts.