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Recombinase-mediated cassette exchange (RMCE) is a powerful method for achieving gene targeting repeatedly at a single mammalian locus. This approach could be applied to the efficient establishment of genetically related cell lines harboring different p53 mutations found in human tumors. To this end we generated a mouse strain called p53 Platform mice (PLF mice), containing PhiC31 integrase-specific attP sequences at the Trp53 locus. The attP sites flank a PGK-neo cassette that has replaced exons 2 to 9 of the endogenous murine p53 gene, generating a null allele. Electroporation of a fluorescence indicator plasmid into embryonic stem (ES) cell lines from PLF mice demonstrated that PhiC31 integrase-mediated cassette exchange (IMCE) can be achieved at > 60% efficiency without selecting against random insertion. To produce somatic cell lines with endogenously controlled expression of mutant p53, we performed IMCE in PLF murine embryonic fibroblasts (MEFs) with plasmid constructs containing human p53 gene sequences carrying specific tumor-associated missense mutations (A138V; G245S). The MEF cell lines produce the expected mutated mRNA transcripts and express p53 protein that is phosphorylated at serine 15 following DNA damage. Within a few weeks one can thus acquire a family of p53 mutant cell lines from the same population of primary cells, but each harboring a different mutation.