Vast majority of cases of Acute Promyelocytic Leukemia (APL) have a translocation t(15;17)(q22;q12-21) which fuses the Promyelocytic gene (PML) on chromosome band 15q22 to the Retinoic acid receptor alpha (RARA) gene at 17q12-21. This PML / RARA fusion is associated with a good response to all- transretinoic acid therapy.