TP53BP1 and cancer: Furthermore, in two non-cancer cell lines, human retinal pigment epithelium (RPE) cells and normal mammary epithelium MCF10A cells, the endogenous 53BP1 protein levels in 2% SDS lysates were diminished 4 h post-IR (Figure 2B, lane 6, 8), consistent with the results from the HeLa and 293T cell lines.