Schematic representation of the DARC genomic locus. DARC is encoded by an ORF contained in two exons separated by one intron. Primer extension and 5′RACE analysis of both human DARC and mouse DARC homolog (Wang Z.X., Lu A.H., Peiper S.C., unpublished observations, September 1995) reveal the presence of an upstream exon containing a methionine translation initiation codon and codons for six additional amino acid residues. An internal initiation codon that may be used in the human gene is not conserved in the mouse gene. The polymorphism responsible for the Fy^a and Fy^b phenotypes is encoded at codon 44.