Key proteins involved in homologous recombination (HR). The main engine of HR is Rad51, which catalyzes the key reactions of synapsis-homology search and DNA strand invasion. Altogether, there are more than 200 proteins involved in different aspects of HR, providing a sufficient number of context-specific factors to explain the extraordinary versatility of HR, i.e., its functions in the repair of double-strand breaks (DSBs), DNA interstrand crosslinks (ICLs), and in resolution of stalled/collapsed replication forks. Any of the proteins are potentially targetable for cancer therapy.