TSPY4 (testis-specific protein Y-linked 4) is a Y-chromosome Y with roles in male germ cell biology and development. Functionally, TSPY4 is expressed as a highly germ cell-specific marker, particularly in spermatogonia during spermatogenesis 1, supporting its involvement in sperm differentiation and proliferation. The protein localizes to chrY and binds histones, suggesting roles in transcriptional regulation within the nucleus. Mechanism: TSPY4 belongs to the TSPY family of Y-encoded genes and may coordinate with related Y-chromosome Y in gene regulation. Related TSPYL family members regulate cytochrome P450 gene expression 2, suggesting similar transcriptional regulatory functions. Disease Relevance: TSPY4 mutations have been identified in juvenile nasopharyngeal angiofibromas (JNA), a locally aggressive head and neck tumor in young males, indicating potential involvement in neoplastic pathogenesis 3. Additionally, misregulation of TSPY4 expression may contribute to male infertility phenotypes 1. Clinical Significance: TSPY4 serves as a sensitive, sperm-specific biomarker for semen exposure in cervicovaginal fluids, detectable up to 72 hours post-exposure and occasionally up to 7 days 45. This application enables objective monitoring of protocol compliance in HIV prevention and contraceptive trials 6. Additionally, TSPY4 detection supports forensic male DNA identification 7 and tracks donor cell persistence in wound healing studies 8.