Isochromosome 11q in acute monoblastic leukemia (M5a).

An isochromosome 11q in a patient with M5a type acute monoblastic leukemia is reported. The leukemic cells had a few azurophilic and alpha-naphthyl butyrate esterase-positive granules in the cytoplasm. Electron microscopy showed indented nuclei, abundant perinuclear fibrous bundles, and small lysosomes which were characteristic of monocytoid cells. A strong inhibitory activity against urokinase was detected in the cell lysates as compared with other leukemic cells. Cytogenetic analysis of the leukemic cells showed the karyotype 47, XY, +i(11q), which has not been observed hitherto in acute monocytic leukemia.