CDY1 is a Y-linked histone acetyltransferase with preference for histone H4 that functions as a transcriptional corepressor and histone reader with specificity for H3K27me3 and H3K9me2/3 marks 1. The gene plays a critical role in late-stage spermatogenesis, with CDY1 expression specifically associated with the presence of mature spermatids and spermatozoa rather than early germ cells 23. CDY1 deletions are implicated in male infertility: loss of both CDY1 copies combined with DAZ deletions significantly increases susceptibility to azoospermia (odds ratio 29.7) and severe oligozoospermia (odds ratio 26) 4. Complete deletion of CDY1a and CDY1b is present in all Sertoli cell-only syndrome patients examined, with no detectable CDY1 mRNA expression in affected individuals 5. Clinically, CDY1 expression assessment by qualitative RT-PCR serves as a predictive marker for sperm presence in testicular tissue, with 98.3% sensitivity 6. However, CDY1 deletions can be vertically transmitted via intracytoplasmic sperm injection, necessitating genetic counseling regarding infertility risk in future male offspring 4. Notably, CDY1 deletions are not universally essential for fertility, as documented cases of natural transmission of large AZFc deletions lacking CDY1 have occurred, suggesting compensatory mechanisms may exist in some individuals 1.