Figure 1: Model for initial response of the cell to a Double Strand Break (DSB). A DNA DSB is initially recognized by the proteins ATM, MDC1, 53BP1, BRCA1 and the damage recognition complex MRN, which consists of the subunits Mre11, NBS1 and RAD50. Those proteins activate complex signal transduction pathways that lead to cell cycle arrest and repair of DNA damage by Homologous Recombination (HR) or Non Homologous End Joining (NHEJ) mechanisms.