BRCA1:BARD1 complex binds RBBP8 (CtIP) phosphorylated by CDK2 at DNA double strand breaks (DSBs), thus forming the so-called BRCA1-C complex. As active CDK2 in complex with CCNA (cyclin A) exists only in the S and G2 phases of the cell cycle, this limits the formation of BRCA1 complex with RBBP8 and the consequent initiation of homologous recombination to S and G2 phases, when the genome is duplicated and sister chromatids are available (Chen et al. 2008, Yun and Hiom 2009). Dissociation of the BRCA1-A complex is necessary for BRCA1 to be able to bind RBBP8 (Coleman and Greenberg 2011). Phosphorylation of BRCA1 by CHEK2 also contributes to the initiation of homologous recombination, with this phosphorylation similarly serving as a molecular clock (Parameswaran et al. 2015).