For the group of AML with defining genetic abnormalities, it now recognizes—apart from known fusions, e.g., PML::RARA, RUNX1::RUNX1T1 or CBFB::MYH11—rearrangements involving KMT2A, MECOM, and NUP98, irrespective of the fusion partner, as disease-defining, even with blast counts below 20% [18]. The gene discussed is RUNX1T1; the disease is acute myeloid leukemia.