APL is characterized by a block in the differentiation of leukemic cells, halting at the promyelocyte stage, and is associated with a signature chromosomal abnormality t(15;17)—a translocation between the long arm of chromosomes 15 and 17 resulting in the fusion of the PML (promyelocytic leukemia) and RARA (retinoic acid receptor α) genes. This evidence concerns the gene RARA and acute promyelocytic leukemia.