MEI1 (meiotic double-stranded break formation protein 1) is a vertebrate-specific gene essential for normal meiotic progression in both male and female gametes. The protein functions primarily in the initiation of programmed DNA double-strand breaks (DSBs) during meiotic prophase I, which are crucial for proper chromosome 22 and recombination 1. In MEI1-deficient mice, spermatocytes exhibit defective DSB formation, as evidenced by reduced histone H2AX phosphorylation levels comparable to SPO11-deficient animals, and failure of RAD51 protein loading onto meiotic chr22 1. Clinically, biallelic MEI1 mutations cause diverse reproductive phenotypes including male non-obstructive azoospermia due to meiotic arrest at the spermatocyte stage 2, and in females, recurrent hydatidiform moles, embryonic arrest, and recurrent implantation failure 3. Recent studies demonstrate that MEI1 mutations lead to aneuploid embryos with abnormal DNA methylation patterns 4. The gene shows testis-specific expression and encodes a protein without significant homology to known proteins, representing a unique vertebrate meiotic regulator 1. These findings establish MEI1 as a critical factor in human fertility, with mutations contributing to both male and female infertility through distinct but related meiotic defects.