PROJECT ABSTRACT Our broad biological goal is to develop a cell-based model that can be used to study the molecular pathogenesis and drug responses due to mutations in ASXL1 (Additional Sex Combs Like 1). Additional Sex Like (ASXL) genes are part of a family of genes that were originally identified as having a role in cell-fate determination in the developing embryo. De novo, truncating mutations ASXL1 cause the pediatric syndrome, Bohring-Opitz Syndrome (OMIM#605039). Mutations in ASXL1 are also driver mutations in acute myeloid leukemia. Despite the clear role in multiple disease pathogenesis, the molecular function of ASXL1 remain unknown and no targeted drugs have been approved for use in patients. This proposal builds on our work which has identified putative epigenetic, RNA and protein biomarkers consistently altered by pathogenic mutations in ASXL1. We will use genome-editing approaches to introduce highly sensitive and endogenous tags to our proteins-of- interest. We will focus on introducing these tags into induced pluripotent stem cell lines as these are able to be differentiated in multiple cell-types that are reflective of key organ systems affected in human disease. These protein tags will allow us to study ASXL1-molecular and cellular function and to develop cell models for future drug screens. This proposal will meet a critical need in rare disease: to develop a cell-based model system to identify drugs that can rapidly identify pathogenic molecular changes observed in ASXL1-mutated diseased cells.