Structure-specific DNA-binding protein involved in DNA repair by promoting RAD51-mediated homologous recombination (PubMed:17996710, PubMed:17996711, PubMed:20871616, PubMed:25288561, PubMed:26323318). Acts by stimulating D-Loop formation by RAD51: specifically enhances joint molecule formation through its structure-specific DNA interaction and its interaction with RAD51 (PubMed:17996710, PubMed:17996711). 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 (PubMed:9396801, PubMed:17996711, PubMed:22375013, PubMed:17996710). 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 (PubMed:27463890, PubMed:27239033, PubMed:32350107). WDR48/UAF1 and RAD51AP1 also have a coordinated role in DNA-binding to promote USP1-mediated deubiquitination of FANCD2 (PubMed:31253762). Also involved in meiosis by promoting DMC1-mediated homologous meiotic recombination (PubMed:21307306). Key mediator of alternative lengthening of telomeres (ALT) pathway, a homology-directed repair mechanism of telomere elongation that controls proliferation in aggressive cancers, by stimulating homologous recombination (PubMed:31400850). May also bind RNA; additional evidences are however required to confirm RNA-binding in vivo (PubMed:9396801). Monomer; elongated monodisperse monomer (PubMed:17996710). Interacts (via C-terminal region) with RAD51; the interaction is direct (PubMed:16990250, PubMed:23754376, PubMed:9396801, PubMed:17996710, PubMed:17996711, PubMed:21903585). Interacts (via SIM motif) with WDR48/UAF1; WDR48/UAF1 and RAD51AP1 cooperate together to stimulate RAD51-mediated homologous recombination (HR) (PubMed:27463890, PubMed:27239033). Interacts (via WVPP motif) with DMC1; the interaction is direct (PubMed:21903585, PubMed:21307306). Interacts with PALB2 (PubMed:20871616). Interacts with RAD52 (PubMed:31400850). Does not interact with DMC1; lack of interaction is caused by the absence of the WVPP motif in this isoform. Colocalizes with RAD51 to multiple nuclear foci (By similarity). Colocalizes with DMC1 on meiotic chromatin (By similarity). Additional isoforms seem to exist. Highly expressed in testis and thymus (PubMed:9396801). Lower levels in colon and small intestine (PubMed:9396801). Little or no expression in spleen, prostate, ovary and peripheral blood leukocytes (PubMed:9396801). Sumoylation with SUMO2/3 by NSMCE2/MMS21 promotes stabilization, possibly by preventing ubiquitination (PubMed:31400850). Sumoylation is required for alternative lengthening of telomeres (ALT) pathway (PubMed:31400850).