Congenital dyserythropoietic anemia (CDA) type I is a rare autosomal recessive macrocytic moderate to severe anemia rarely associated with distal skeletal deformities. Erythroid bone marrow findings are pathognomonic with internuclear chromatin bridges, spongy heterochromatin, and invagination of the nuclear membrane. Our and others studied suggested erythroid S phase arrest. We recently identified the gene mutated in this disease, CDAN1 and named its ubiquitous encoded protein codanin-1. The function of this protein remains unknown. The aim of the present study was to determine the sub cellular localization of codanin-1. Human erythroid cultures as well as Hela, K562 and osteodarcoma cell line (U2OS) were grown. A polyclonal anti-codnain-1 antibody was generated by immunizaing rabbits with a synthetic 17 aa peptide. The anti-sera was affinity-purified against the immunizing peptide. Western blots, immunoflurescence stains and immunogold electromicroscopic studies were performed. Western blots of human erythroid cultures as well as HeLa, K562 and osteosarcoma cell lines identified one expected band of 130kD. Fluorescent and confocal microscopic studies of osteosarcoma and HeLa cells using anti-codanin-1 antibody showed that codanin-1 was an abundant nuclear protein. Immunogold electron microscopic studies with anti codanin-1 antibodies demonstrated that coodanin-1 is located within the heterochromatin. Future studies will focus on the role of codanin-1 within the heterochromatin.