The molecular mechanism defining susceptibility of normal cells to oncogenic transformation may be a valuable therapeutic target. We characterized the cell of origin and its critical pathways in MN1 leukemias. Common myeloid (CMP), but not granulocyte-macrophage progenitors (CMP) could be transformed by constitutively overexpressed MN1. Complementation studies of CMP-signature genes in GMPs demonstrated that leukemogenicity of MN1 required the MEIS1/abdB-like HOX protein complex. Colocalization studies by ChIP-seq identified common chromatin targets of MN1 and MEIS1 that were associated with open chromatin and transcriptional activation. Transcriptional repression of MEIS1 target sites in established MN1 leukemias had antileukemic activity. As MN1 relies on but can not activate expression of MEIS1/abdB-like HOX proteins, transcriptional activity of these genes determines which cell is the cell of origin in MN1 leukemia.