CDY1B (chrY Y-linked 1B) is a Y-chromosome Y located in the azoospermia factor c (AZFc) region that plays a role in spermatogenesis. As a single-copy gene with a proximal homolog (CDY1a), CDY1b exists in multiple copies on the Y chrY 1. The gene is expressed in testicular tissue 2, and its protein product contains a chrY, suggesting involvement in chrY-related functions during spermatogenesis. CDY1b deletion shows population-specific associations with male infertility. In Tunisian populations, CDY1b deletion (without CDY1a deletion) was significantly more frequent in infertile patients compared to fertile men (7% vs 1.7%, p=0.02) 3. Combined gr/gr-DAZ2-DAZ4-CDY1b deletions constitute high-risk factors for infertility in this population 2. In contrast, some studies found limited fertility impact; European populations showed no consistent relationship between CDY1b presence/absence and spermatogenic failure across Y-chromosome Y 4. Chinese Han populations demonstrated that gr/gr deletions involving CDY1b with DAZ1/DAZ2 confer significantly higher risk than other deletion subtypes 5. CDY1b double deletions (with CDY1a) appear most severe, being present in all patients with Sertoli cell-only syndrome non-obstructive azoospermia 6. Clinically, CDY1b deletion status may be relevant for fertility assessment, particularly in non-European populations, though genetic counseling should account for Y-chrY haplogroup background.