In the S/G2 phase of the cell cycle, CDK2:CCNA (CDK2:cyclin A) complex and CtIP (RBBP8) are recruited to DNA double strand breaks (DSBs) by the MRN complex. RBBP8 interacts with NBN (NBS1) and RAD50 subunits, while CDK2:CCNA interacts with the MRE11A subunit of the complex (Buis et al. 2012). RBBP8 functions as a homotetramer (Davies et al. 2015).