Myeloproliferative disease of childhood associated with a trisomy 21 clone.

Myeloproliferative disease of childhood is frequently associated with chromosomal anomalies, usually of the C group. Clinical features are similar to those of the juvenile type of chronic myeloid leukemia. A child with this disease is described. Marked myeloid proliferation, anemia, thrombocytopenia and hepatosplenomegaly were present; leukocyte alkaline phosphatase and fetal hemoglobin were moderately elevated. Chromosome analysis of bone marrow cells revealed a mosaicism 47,XX,+21/46,XX. Down's syndrome was ruled out by the child's normal phenotype and dermatoglyphic analysis. The cytogenetic finding is probably evidence for the clonal origin of the trisomy 21 cell line.