| E2F1 induced MRN foci are similar to irradiation induced foci ( IRIF ) that result from double strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci , do not form in NBS cells , do form in AT cells and do not correlate with cell cycle entry . ^^^ In fact , we find that in human fibroblasts deregulated E2F1 causes a G 1 arrest , blocking serum induced cell cycle progression , in part through an Nbs1 / 53BP1 / p53 / p21 ( WAF1 / CIP1 ) checkpoint pathway . ^^^ |