A high level of fetal hemoglobin was found in an 8-yr-old boy without any hematologic disorders except for a moderate anemia. The absence of hemoglobin abnormalities in the parents led us to suspect a latent malignant disease that, on follow-up, was confirmed to be myelomonocytic leukemia. Hemoglobin biosynthetic studies provided evidence of unbalanced synthesis of globin subunits by reticulocytes, while the production of non-alpha chains was equal to that of alpha chains in bone marrow cells. The expression of red cell antigen i was increased, while those of I, A, and A1 antigens were found to decrease progressively. Two populations of erythrocytes, A-positive and A- negative, were distinguished and could be separated by differential agglutination. Unbalanced globin chain synthesis, increased fetal hemoglobin, and antigenic changes of the membrane were shown to be restricted to the A-negative population. The biologic data were not entirely consistent with a genuine reversion to fetal erythropoiesis. The question remains of a polychromosomal lesion of either quiescent F cells or adult stem cells.

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