SUMO E3 ligase that acts as a regulator of crossing-over during meiosis: required to couple chromosome synapsis to the formation of crossover-specific recombination complexes. Localizes to recombination sites and stabilizes meiosis-specific recombination factors, such as MutS-gamma complex proteins (MSH4 and MSH5) and TEX11. May mediate sumoylation of target proteins MSH4 and/or MSH5, leading to enhance their binding to recombination sites. Acts as a limiting factor for crossover designation and/or reinforcement and plays an antagonist role with CCNB1IP1/HEI10 in the regulation of meiotic recombination. Protein modification; protein sumoylation. Associates to the synaptonemal complex. Localizes to a minority of double-strand breaks (DSBs) sites. Marks crossover sites during midpachynema. Specifically expressed in meiocytes of the gonads. In spermatocytes, first detected at the transition from leptonema to zygonema, localizing specifically to initial sites of homolog synapsis. The number of synaptonemal complex-associated Rnf212 increases as synapsis proceeds. Excluded from unsynapsed homolog. In early pachynema, as cells complete synapsis, detected as a punctate pattern of irregular foci along the synaptonemal complexes. Also localizes to the synapsed pseudoautosomal regions of the X-Y chromosomes. After early pachynema, protein levels strongly drop. By midpachynema, Rnf212 foci only remain at sites where crossovers form. Remaining foci disappear in late pachynema before the disassembly of synaptonemal complexes at diplonema and are not detected in early diplotene-stage cells in which homologs begin to desynapse. Pattern in fetal oocytes is very similar, except that the late-stage Rnf212 foci are still detected in nuclei in both the late-pachytene and early diplotene stages (at protein level). Both male and female mice are sterile. Males do not make sperm and have much smaller testes, a characteristic of mutants with meiotic defects. Testes show an absence of post-anaphase I cells, indicating loss of spermatocytes at this stage (stage XII seminiferous tubules). In females, ovary size is similar to that of wild-type animals, and high numbers of oocytes are present in mature animals. Complete synapsis is achieved but crossover complexes are absent. Rnf212 is haploinsufficient: although herozygous males are fertile and have wild-type sperm counts, they show significantly fewer Mlh1 foci and fewer chiasmata. Haploinsufficiency suggests that Rnf212 is a limiting factor for crossover designation and/or reinforcement.