Because Brca1−/− (knockout) mice are embryonic lethal and p53−/− mice develop and succumb from tumors (e.g., lymphoma, sarcoma, etc.)at young ages, the present study utilized genetically engineered mouse strains that express LoxP recombination sites within the Brca1 and p53 alleles, thus allowing conditional inactivation of each allele upon exposure to Cre-recombinase [31]. The gene discussed is TP53; the disease is sarcoma.