The low incidence erythroid antigens, BOW, NFLD, and Fra, are excellent Diego blood group system candidates based on a variety of serological, biochemical, and molecular data. Diego system antigens are carried on the erythroid membrane protein band 3, and are controlled by 'SLC4A1' (solute carrier family 4, anion exchanger member I gene). Band 3 contains two functionally distinct domains; an N-terminal cytoplasmic domain and a C-terminal transmembrane domain. DNA from controls and individuals segregating for BOW, NFLD, and Fr a, was amplified by PCR and screened for polymorphisms by single strand conformation polymorphism analysis (SSCP). Only exons corresponding to the transmembrane domain of band 3 (exons 11-20 of 'SLC4A1') were analyzed. DNAs displaying SSCPs were subjected to sequencing. DNA from BOW+ individuals displayed a SSCP in exon 14 and exhibited a single C [right arrow] T mutation, corresponding to a Pro561Ser substitution in band 3. DNA from all tested NFLD+ individuals displayed SSCPs in bothexon 12 and 14. Subsequent DNA sequencing identified A [right arrow] T and C [right arrow] G mutations in exons 12 and 14 that give rise to Glu429Asp and Pro561Ala substitutions in band 3, respectively. Exon 13 SSCPs were evident in the DNA of Fr(a+) individuals tested. DNA sequencing revealed a G [right arrow] A mutation underlying a Glu480Lys substitution. We conclude that point mutations in 'SLC4A1' representing amino acid substitutions in the erythroid protein band 3 account for all three antigens, In light of these findings, the International Society of Blood Transfusion (ISBT) Working Party on Terminology for Red Cell Surface Antigens has granted BOW, NFLD and Fra, Diego blood group system status and designated them DI15, DI16, and DI20 respectively.