TP53 and breast carcinoma: However, the results of this report suggest that IR-induced G2/M cell-cycle arrest as well as the regulation of Rac1 on the IR-induced G2/M checkpoint response is apparently independent of p53, as among the four breast cancer cell lines used for the studies, MDA-MB-231 and T47D cells express mutant p53 [56], whereas MCF-7 and ZR75-1 express wild-type p53.