The Colton (CO) blood group system consists of four antigens, Co a , Co b , Co3, and Co4, located on aquaporin-1 (AQP1), with Co a highly prevalent in all populations (99.8%). The Colton null phenotype, Co(a-b-), is very rare, and individuals with this phenotype lack the high-prevalence antigen Co3. To date, only six Co(a-b-) probands have been reported and four silencing alleles characterized. We identified an AQP1-null allele in a white woman with anti-Co3 caused by deletion of a G at

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... de 601 (nt601delG) that results in a frameshift and premature termination (Val201Stop). Available family members were tested for the allele. Although anti-Co3 has been associated with mild to severe hemolytic disease of the fetus and newborn, the antibody was not clinically significant as evidenced by a low titer and delivery of asymptomatic newborns with moderate to weakly positive direct antiglobulin tests for all four pregnancies. Immunohematology 2013;29:1-4. Proband NT Fig. 1 DNA sequence electropherogram of AQP1 exon 3. (A) Proband (AQP1 null). (B) Consensus (conventional AQP1). The proband was homozygous for a G nucleotide deletion at position 601, which is predicted to cause a premature stop (Val201Stop) in the protein.