Structure-specific DNA-binding protein involved in DNA repair by promoting RAD51-mediated homologous recombination. Acts by stimulating D-Loop formation by RAD51: specifically enhances joint molecule formation through its structure-specific DNA interaction and its interaction with RAD51. Binds single-stranded DNA (ssDNA), double-stranded DNA (dsDNA) and secondary DNA structures, such as D-loop structures: has a strong preference for branched-DNA structures that are obligatory intermediates during joint molecule formation. Cooperates with WDR48/UAF1 to stimulate RAD51-mediated homologous recombination: both WDR48/UAF1 and RAD51AP1 have coordinated role in DNA-binding during homologous recombination and DNA repair. WDR48/UAF1 and RAD51AP1 also have a coordinated role in DNA-binding to promote USP1-mediated deubiquitination of FANCD2. Also involved in meiosis by promoting DMC1-mediated homologous meiotic recombination. Monomer; elongated monodisperse monomer (By similarity). Interacts (via C-terminal region) with RAD51; the interaction is direct (PubMed:9192668). Interacts (via SIM motif) with WDR48/UAF1; WDR48/UAF1 and RAD51AP1 cooperate together to stimulate RAD51-mediated homologous recombination (HR) (By similarity). Interacts (via WVPP motif) with DMC1; the interaction is direct (By similarity). Interacts with PALB2. Interacts with RAD52 (By similarity). Colocalizes with RAD51 to multiple nuclear foci (PubMed:9192668, PubMed:21307306). Colocalizes with DMC1 on meiotic chromatin (PubMed:21307306). Most abundantly expressed in testis (PubMed:9192668). Also expressed in spleen, thymus and bone marrow (PubMed:9192668). Not detected in heart, kidney or liver (PubMed:9192668). Highly expressed in 15 to 21 days postpartum (dpp) testes. Sumoylation with SUMO2/3 by NSMCE2/MMS21 promotes stabilization, possibly by preventing ubiquitination.