MCM9 (minichromosome maintenance 9 homologous recombination repair factor) functions as a critical component of DNA repair mechanisms and reproductive biology. MCM9 forms a heterodimeric complex with MCM8 that acts as a 3'-5' DNA helicase essential for repairing double-strand breaks and DNA interstrand cross-links through homologous recombination 1. The MCM8-MCM9 complex also plays a crucial role in DNA mismatch repair by unwinding mismatch-containing DNA strands and recruiting MLH1 to chr6 2. During gametogenesis, MCM9 is predominantly expressed in spermatogonial stem cells and spermatogonia, where it maintains genomic integrity through both homologous recombination and mismatch repair pathways 2. Loss-of-function mutations in MCM9 cause primary ovarian insufficiency (POI) in females, characterized by premature ovarian failure before age 40 34. In males, MCM9 deficiency leads to non-obstructive azoospermia and Sertoli cell-only syndrome due to impaired DNA damage repair during spermatogenesis 2. Beyond reproductive disorders, biallelic MCM9 variants are associated with increased cancer risk, particularly colonic polyps, gastric cancer, and early-onset colorectal cancer 5. MCM9 also functions in mitophagy regulation, protecting vascular health by promoting mitochondrial quality control in response to nitric oxide signaling 6.