The RA-target gene RARβ plays a crucial role in mediating the growth-inhibitory and tumor suppressive effects of retinoids in various cancer cells [84–87], and RARβ is silenced in many tumors [84, 87, 88] and myeloid leukemias [89, 90] including APL [13]. This evidence concerns the gene RARB and acute promyelocytic leukemia.