System used to describe categories of acute leukemia. Acute myelocytic leukemiaÑM1, acute myelocytic leukemia without differentiation; M2, acute myelocytic leukemia with differentiation (predominantly myeloblasts and promyelocytes); M3, acute promyelocytic leukemia; M4, acute myelomonocytic leukemia; M5, erythroleukemia. Acute lymphocytic leukemiaÑL1, predominantly ÒsmallÓ cells (twice size of normal lymphocyte), homogeneous population; L2, larger than L1, more heterogeneous population; L3, Òburkitt-likeÓ large cells, vacuolated abundant cytoplasm.