mice are exposed to tamoxifen following exposure to tamoxifen the cre er t2 fusion protein is activated and the conditional allele converts to a mutant allele and in various embodiments the conversion to the mutant allele is irreversible with deletion of the wild type allele in this manner a mouse line containing an otherwise lethal acvr1 mutation can be maintained essentially indefinitely producing the desired genetic lesion and accompanying phenotype whenever desired in various embodiments a genetically modified mouse comprising the acvr1 coin allele is made by modifying a mouse es cell to contain the coin allele and modifying the same es cell to contain a gene encoding the tamoxifen inducible cre er t or cre er t2 and using the es cell as a donor cell to make a mouse that contains the coin allele and the modified cre gene all of the references cited herein are hereby incorporated by reference in order to engineer a mouse model of fibrodysplasia ossificans progressiva fop the r206h u201c classic fop u201d mutation of human acvr1 shore et al 2006 was engineered into the corresponding mouse gene acvr1 this mutation has already been modeled non conditionally in the mouse but the resulting chimeric mice arising from blastocyst microinjection of the targeted es cells were.