Homology recognition and DNA-strand invasion ensure faithful homolog pairing and segregation during the first meiotic division 1 . RAD51 and DMC1 recombinases catalyze these steps 2 , with BRCA2 promoting their assembly into nuclear foci 3 . The recently identified human SWS1-SWSAP1 complex, related to the Shu complex in yeast, promotes RAD51 focus formation in cell lines 4,5 . We show here that mouse SWS1-SWSAP1 is critical for meiotic homologous recombination (HR) by promoting the assembly of RAD51 and DMC1 on early recombination intermediates. Absence of the complex perturbs meiotic progression in males and females and both sexes are sterile, although a fraction of meiocytes form crossovers. Remarkably, loss of the DNA damage checkpoint kinase CHK2 rescues fertility specifically in females without rescuing crossover numbers. Unlike the Shu complex, the BRCA2 C terminus (known to be required for RAD51 stabilization 6,7 ) is dispensible for RAD51 and DMC1 focus formation. However, concomitant loss of the BRCA2 C terminus aggravates the meiotic defects in Shu mutant spermatocytes. These results point to a complex interplay of factors that ensure recombinase function and hence meiotic progression in the mouse.Regulating the assembly, stabilization, and disassembly of nucleoprotein filaments of RAD51 and its meiosis-specific paralog DMC1 is critical for productive meiotic HR 2 .Transgenic mouse studies have shown that BRCA2, a multi-domain protein with several interaction sites for both recombinases 7-9 is necessary for RAD51 and DMC1 focus formation during meiosis, such that BRCA2 loss causes meiotic arrest and sterility 3 . Other proteins are also expected to play a role in this process, including the canonical RAD51 paralogs, but understanding their meiotic role has been hampered by the embryonic lethality of mutants 10 .More recently, "Shu" complexes have been identified in several organisms; these interact with RAD51 and RAD51 paralogs and modulate RAD51 11 . Shu complexes are comprised of a defining member with a conserved Zn-coordinating motif and one or more members with RAD51-like structural motifs 4,5,[11][12][13] . In budding yeast, mutations in any of the four subunits of the Shu complex suppress the slow growth and hydroxyurea sensitivity ("Shu") of sgs1 and top3