USP9X and breast cancer: Consistent with known haploinsufficient tumor suppressor activity for multiple genes on the TGFβ pathway6, it is important to note that hemizygous loss is common for many of these genes in breast cancer (SMAD4—24%, SMAD2—22%, USP9X—21%, TGFBR1—19%, BMPR1A—18%, and ACVR2A— 18%)25.