Even more interestingly, a conditional knock-in mouse in whom the human BCR-ABL1 cDNA was knocked into the endogenous mouse Bcr locus so that it could be conditionally expressed with different tissue-specific Cre transgenes under the added control of the native Bcr regulatory elements, was found not to develop leukemia during its lifetime, despite expression of a constitutively active BCR-ABL1 tyrosine kinase was observed in the hematopoietic progenitors [31]. This evidence concerns the gene BCR and leukemia.