TP53 and breast cancer: Furthermore, we observed that S15 phosphorylation of p53 (pp53 S15) was sustained until 24 hours post incubation of HMECs with exosomes from all three breast cancer cell lines, MDA-MB-231, 47DA18 and MCF7 cells, respectively (Fig. 6 B, lanes 2 to 4 vs. lane 1).