TP53 and breast cancer: However, XAF1 p.E134* does not seem to be an important modifier of TP53 p.R337H-driven breast carcinogenesis because only 77% (Group 1, equally distributed in the three subregions, 76% in C1, 79% in C2, and 75% in C3) and 75% (Group 3) of the BC cases harbored both variants, whereas newborns with unknown cancer history presented 69% with both haplotypes [8] and 72.3% of R337H carriers together with p.E134* in non-affected participants (i.e., any cancer) in the present study.