TP53 and cancer: To this end, we used previously established strains of genetically engineered mice harboring LoxP-flanked Brca1[16] in conjunction with mice harboring LoxP-flanked p53[26] or a missense gain-of-function mutant allele of p53[27] that is analogous to the R175H hotspot mutation frequently identified in human cancers [28], [29], [30].