ABSTRACT Apoptosis, a highly regulated process of programmed cell death, is essential for maintaining tissue homeostasis and development. Intrinsic apoptosis is regulated by the competitive interactions between anti- apoptotic and pro-apoptotic BCL-2 family members containing the BH3 helix or BH3 binding groove. MCL1 is an anti-apoptotic BCL-2 protein at the center of the intrinsic apoptotic pathway. Cells require strict control over MCL1 expression to maintain homeostasis. Aberrant MCL1 expression facilitates tumor progression and chemoresistance. Understanding the mechanisms that regulate MCL1 expression will improve MCL1 targeting in anti-cancer therapy. For instance, a growing understanding of the degradation factors affecting MCL1 has provided key insights regarding cellular response to prolonged mitosis. Yet, little is known regarding the regulation of MCL1 mRNA. While searching for putative RNA regulatory proteins we observed that polypyrimidine tract binding protein 1 (PTBP1), a master regulator for post-transcriptional control of gene expression through mRNA splicing, translation, turnover and localization, associated with MCL1. PTBP1 had previously come to our attention as a putative rBH3 containing protein that may directly interact with MCL1. Like MCL1, PTBP1 has been shown to be a critical factor in cellular differentiation, a mitotic regulator, and exhibits significant overexpression across the cancer landscape. Based on these data and our preliminary findings we propose in this grant to: (1) characterize post-transcriptional control of MCL1 mRNA by PTBP1, (2) characterize the direct interaction between PTBP1 and MCL1, and (3) characterize the effect of PTBP1 association with MCL1 protein and mRNA on cellular response to DNA damage and mitotic stress. Understanding the crosstalk between PTBP1 and MCL1 will advance the basic research in the field of apoptosis during normal development and cancer.