M1AP (meiosis 1 associated protein) is essential for male fertility, functioning as a critical regulator of meiotic recombination during spermatogenesis. M1AP interacts with ZZS proteins (SHOC1/TEX11/SPO16) to promote DNA class I crossover formation and facilitate homologous chromosome 2 during meiosis I 1. The protein plays a non-essential but functionally important role as a meiotic recombination enhancer; while M1AP deficiency does not completely prevent meiosis, it results in reduced recombination intermediates and crossover formation 1. Biallelic loss-of-function variants in M1AP cause severe male infertility through distinct meiotic impairments. Men carrying M1AP mutations exhibit predominant metaphase I arrest with rare haploid spermatids, distinguishing this phenotype from ZZS protein deficiency which causes earlier meiotic arrest 1. M1AP mutations are a relatively frequent cause of non-obstructive azoospermia (NOA) and severe oligozoospermia, with homozygous frameshift and missense variants identified across multiple infertile cohorts 2. Individual missense variants alter M1AP protein dynamics without causing gross misfolding, suggesting loss-of-function through disrupted meiotic interactions 3. Despite severe phenotypes, medically assisted reproduction offers reproductive options for affected men 1. Notably, M1AP mutations represent novel paternal genetic factors contributing to early embryonic arrest in assisted reproduction 4.