The specification of megakaryocytic (Mk) or erythroid (E) lineages from primary human megakaryocytic-erythroid progenitors (MEP) is crucial for hematopoietic homeostasis, yet the underlying mechanisms regulating fate specification remain elusive. In this study, we identify RUNX1 as a key modulator of gene expression during MEP fate specification. Overexpression of RUNX1 in primary human MEP promotes Mk specification, while pan-RUNX inhibition favors E specification. Although total RUNX1 …