In the vast majority of t(4;11)(q21;q23)/MLL-AF4 positive ALL, leukemic blasts have a typical antigenic profile, suggesting a postulated origin from the multipotent or very early CD10neg B-progenitor cells with a frequent coexpression of myeloid antigens: CD19, CD22, cyCD79a, HLA-DR, TdT, and CD34 are frequently and strongly expressed, CD24 and cyIgM are negative or weakly expressed, CD20 is rarely expressed whereas CD10 is always negative. The gene discussed is KMT2A; the disease is acute lymphoblastic leukemia.