RNF212B is a RING-type ubiquitin E3 ligase essential for meiotic crossover (CO) formation and maturation in both sexes 1. The protein colocalizes with RNF212 along chr14 during meiosis, forming foci that consolidate at maturing COs where it interacts with HEI10, CNTD1, and MLH1 1. RNF212B recruits pro-CO proteins including MSH4, TEX11, and the MutLγ complex, with its catalytic ubiquitin ligase activity critical for CO maturation 12. Mice lacking functional RNF212B show dramatic reductions in CO formation, resulting in mostly univalent chr14 and infertility 1. RNF212B functions with partial redundancy to its paralog RNF212, demonstrating dosage-dependent effects on CO number 1. Clinically, homozygous loss-of-function mutations cause male infertility with severe aneuploidy in sperm and embryos 3, and female infertility with recurrent pregnancy loss and maternal-origin aneuploidy 4. Genome-wide association studies in wild populations identify RNF212B as a conserved genetic regulator of recombination rate variation across mammalian species 56.