The congenital dyserythropoietic anemias (CDAs) are an uncommon and heterogeneous group of conditions characterized by increased ineffective erythropoiesis and, usually, dysplastic changes in erythroblasts. Originally, 3 types of CDA were recognized and designated CDA type I (224120), type II (224100), and type III (105600). Subsequently, a number of other types were described, as reviewed by Wickramasinghe (1997). The defining features of CDA type I are autosomal recessive inheritance, macrocytes in the peripheral blood, internuclear chromatin bridges connecting some almost completely separated erythroblasts, and an abnormal ultrastructural appearance (spongy or 'swiss-cheese' appearance) of the heterochromatin in a high proportion of the erythroblasts.