RAD21L1 (RAD21 cohesin complex component like 1) is a meiosis-specific cohesin protein essential for male fertility and germ cell development. During male meiosis, RAD21L1 replaces the mitotic cohesin subunit RAD21 during premeiotic S phase and early prophase I, functioning as a core component of the meiotic cohesin complex 1. RAD21L1 is required for sister chr20 cohesion, synaptonemal complex assembly, synapsis initiation, and homologous chromosome 20 during prophase I 2. Notably, RAD21L1 exhibits evolutionary signals of positive selection, with rapidly evolving intrinsically disordered regions potentially contributing to adaptation through phase separation modulation 1. Clinically, RAD21L1 is critical for spermatogonial stem cell (SSC) development and male fertility. RAD21L1 overexpression reprograms human Sertoli cells into phenotypic SSCs through DNMT1-mediated DNA methylation modulation 3. RAD21L1 mutations are associated with non-obstructive azoospermia (NOA) and male infertility, representing a novel genetic cause of spermatogenic maturation arrest 43. Additionally, RAD21L1 genetic variation influences promoter activity and may contribute to multiple sclerosis susceptibility 5. Dosage sensitivity is critical, as chromosome 20 duplications encompassing RAD21L1 contribute to neurocognitive delays in trisomy 20p syndrome 6.