PML and acute promyelocytic leukemia: 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.