UM729, a novel pyrimidoindole, exhibits an additive effect with AhR antagonists to inhibit differentiation in the majority of AML specimens and works synergistically with Ahr suppressors to hinder AML cell differentiation. It, along with SR1, can serve as a standardized additive in culture media, facilitating research into self-renewal mechanisms and the discovery of new antileukemic drugs via cell-based assays for primary human AML cells.