ATRA in treating APL. (A) Isomers of retinoic acid. (B) ATRA induces terminal differentiation of abnormal promyelocytes in vivo. On day 30 of treatment, Auer bodies (arrow) are found in neutrophils circulating in the peripheral blood, indicating these cells are derived from leukemic promyelocytes. (C) ATRA treatment leads to elimination of PML-RARα–positive cells revealed by detection of minimal residual disease (MRD) using quantitative real-time RT-PCR for assessment of PML-RARα transcript. Effects of ATO and ATRA in combination with ATO are also shown. Stains were analyzed using an Olympus BX51 research microscope equipped with a 100×/1.3 NA oil objective, and images were processed using Adobe Photoshop CS.