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English: The three BRCA1 complexes in humans. The three human complexes in which the BRCA1–BARD1 heterodimer was found are called the A, B, and C complexes. The A complex is involved in DNA repair via homologous recombination. The heterodimer is needed to load RAD51 on single-stranded DNA, where it then catalyzes the strand invasion. The B complex has a role in the G1/S cell cycle checkpoint, and the C complex is involved in the G2/M checkpoint and 5′ end resection, which is the initial step after a DNA double strand break. See the text for details. (Proteins with a known homolog in Arabidopsis are colored, and the names are written in bold; ABRA1, Abraxas1; BARD1, BRCA1 associated RING domain protein 1; BRCA1, breast cancer susceptibility gene 1; BRCA2, breast cancer susceptibility gene 2; BRCC36, BRCA1/BRCA2 containing complex subunit 36; BRCC45, BRCA1/BRCA2 containing complex subunit 45; CtIP, CtBP interaction partner; FANCJ, Fanconi anemia complementation group J; MLH1, mutL Homolog 1; MRE11, meiotic recombination 11 homolog A; MSH6, mutS Homolog 6; NBA1, new component of the BRCA1 A complex; NBS1, Nijmegen breakage syndrome 1; PALB2, partner and localizer of BRCA2; RAD50, radiation sensitive 50; RAD51, radiation sensitive 51; TopBP1, topoisomerase 2 binding protein 1)