TP53 and Li-Fraumeni syndrome: However, it is not clear whether the risk of breast cancer among women with Li-Fraumeni syndrome is due to reduced activity of p53 or the increased risk of losing the wild-type allele of TP53. As levels of radiation-induced apoptosis in Trp53+/- mouse mammary epithelium were intermediate to the responses in Trp53+/+ and Trp53-/- tissues (Figure 6a), Trp53+/- mammary tissues are haploinsufficient with respect to apoptosis.