ABSTRACT
Thrombocytopenia and absent radii (TAR) syndrome is a congenital defect with osseous abnormalities and thrombocytopenia. It is inherited as an autosomal recessive trait, but the mechanism of thrombocytopenia in this disorder is not clear. We have had the opportunity to study the mechanism of thrombocytopenia in an infant with TAR syndrome. The infant had normal levels of thrombopoietin and megakaryocyte colony-stimulating activity in spite of marked thrombocytopenia. However, the megakaryocyte progenitor cells in the bone marrow produced abnormal colonies with increased numbers of megakaryocytes per colony and small megakaryocytes similar to the small megakaryocyte seen in vivo. These findings suggest that the TAR syndrome in this infant is due to a failure in the production of thrombopoietin or to an abnormal progenitor cell with a maturational defect.