The presynaptic phase of homologous DNA pairing and strand exchange during homologous recombination repair (HRR) begins with the displacement of RPA from ssDNA (Thompson and Limoli 2003) by the joint action of RAD51 and BRCA2. CHEK1-mediated phosphorylation of RAD51 and BRCA2 (Sorensen et al. 2005, Bahassi et al. 2008) is needed for BRCA2-mediated nucleation of RAD51 on 3'-ssDNA overhangs, RPA displacement and formation of RAD51 nucleofilaments (Yang et al. 2005, Jensen et al. 2010, Liu et al. 2010, Thorslund et al. 2010). Invasive RAD51 nucleofilaments are stabilized by the BCDX2 complex composed of RAD51B, RAD51C, RAD51D and XRCC2 (Masson et al. 2001, Chun et al. 2013, Amunugama et al. 2013).