Reagents ATO answer was obtained from the pharmacy of our hospital. We believed that signaling pathways, along with ROS generation, may be involved with ATO induced apoptosis in APL cells. The mitochondrial apoptotic pathway is controlled by three main anti-apoptotic GW0742 proteins, Bcl 2, Bcl xL, and Mcl 1, which block the functions of the proapoptotic proteins Bax and Bak. Recently we discovered that APL NB4 cells expressed Bcl 2 and Mcl 1, although not Bcl xL. Mcl 1 has been found to play a critical position in the regulation of neutrophil apoptosis and to be needed for the survival of hematopoietic stem cells. Consequently, Mcl 1 might play an essential part in protecting cells from apoptotic death in APL cells. Activated PI3K/AKT/mTOR signaling occurs in AML cells. Activated mTOR signaling was found to promote cell survival by improving translation of proteins, including Mcl 1. Mcl 1 is a short lived protein because of rapid degradation after post transcriptional phosphorylation by AKT and ERK kinases. It’s been found that ATO therapy diminished AKT levels in APL cells and that inhibitors of ERK and AKT enhanced ATOinduced apoptosis in non APL leukemia cells. Recently, it has been discovered that activated glycogen synthase kinase Eumycetoma 3 phosphorylated Mcl 1 and resulted in proteasomal degradation of Mcl 1. We thought that Mcl 1 levels are regulated by ATO and that Mcl 1 might have a role in ATO induced apoptosis of APL cells, since GSK3 is inhibited by AKT. APL NB4 cells, but maybe not non APL HL 60 cells, respond to apoptosis induction subsequent ATO treatment at therapeutic concentrations. We compared the regulation of Mcl 1 protein levels as a result of ATO treatment in NB4 and HL 60 cells and found that the Mcl 1 protein was decreased in NB4 histone deacetylase HDAC inhibitor cells, but not in HL 60 cells. The mechanism of Mcl 1 down-regulation by ATO treatment in NB4 cells was investigated by analyzing the signaling pathways of ERK, mTOR, AKT and GSK3B. We found that ATO decreased Mcl 1 levels by activating GSK3B by inhibition of AKT and ERK in APL cells. The role of decreased Mcl 1 levels in ATO induced apoptosis was examined in HL 60 cells by silencing Mcl 1 using siRNA. We tested the combined apoptotic effects of ATO having an AKT or an ERK inhibitor in HL 60 cells and investigated the potential mechanisms of apoptosis induction of these combinations, to enhance the apoptotic effects of ATO in non APL cells. We discovered that sorafenib, a Raf inhibitor, decreased intracellular reduced glutathione levels, decreased Mcl 1 levels, and augmented ATO induced ROS generation and apoptosis in HL 60 cells as well as in major AML cells. Our data suggest that therapy with ATO plus sorafenib should benefit non APL AML patients. U0126, mek inhibitors and PD184352, and the Raf inhibitor sorafenib were obtained from LC Laboratories.