Microchromosomes have been observed in direct bone-marrow chromosome preparations in 9 patients. Seven of these patients had overt myelomonocytic leukemia, one had a preleukemic syndrome initially and subsequently had acute leukemia, and one is believed to be in the preleukemic phase of myelomonocytic leukemia. I t is suggested that the microchromosomes in these cases are marker chromosomes which identify a clone or clones of leukemic cells. Microchromosomes appear to be frequent findings i n myelomonocytic leukemia.
ICROCHROMOSOMES, OR MINUTE CHROMO-M somes, are very small chromatin structures which range from approximately half the size of a G-group chromosome (the smallest normal human chromosomes) to about that of a naked centromere. Microchromosomes are an abnormal finding in man but may be part of the normal chromosome constitution of mosses, certain higher plants, insects, reptiles, birds, and a few species of mammals.3~ 1 0 9 1 * , 2 3 Microchromosomes similar to those which occur naturally have been observed in virus-induced sarcoma of micejv 9, 12, 16-and in a few types of human tumors including neuroblastoma, medulloblastoma, bronchial carcinoma, and metastatic carcinoma.2,13,15, 1 7 . 2 0 Although small chromosomes occur in human leukemia (for example, the Philadelphia 1 chromosome in chronic granulocytic leukemia), microchromosomes have been reported previously in only 2 patients with leukemia.3~ 22 We have observed microchromosomes in direct bonemarrow chromosome preparations in 9 additional cases. Eight of these patients had myelomonocytic leukemia, and the ninth is believed to be in the preleukemic phase of myelomonocytic 1e~kernia.l~ From the Mayo Clinic and Mayo Foundation: Department of Clinical Pathology (Hematology).