Several studies have shown that a natural RAR agonist, all-trans retinoic acid (ATRA), or both a natural RAR and RXR agonist, 9-cis retinoic acid (9cRA; an isomer of ATRA), inhibited cell proliferation and ACTH secretion in ACTH-secreting tumor cells [4, 5], or reduced the tumor size in dogs with Cushing’s disease [6], suggesting that RA might exert novel therapeutic effects for Cushing’s disease [7]. Here, RARA is linked to neoplasm.