Glycophorin C (GPC) and glycophorin D (GPD) are highly glycosylated integral membrane proteins of human erythrocytes encoded by the same gene and associated with expression of Gerbich blood group system antigens. GPC/D deficiency (the Leach phenotype) is a rare condition usually found after identification of Gerbich blood group system antibodies in persons undergoing prenatal or pretransfusion evaluation. In all cases, the Leach phenotype has been associated with elliptocytosis. Characterization of the molecular basis of this phenotype in three previously uninvestigated families has shown that the most common genetic basis of GPC/D deficiency is deletion of exons 3 and 4 of the GPC gene. However, in one family, the Leach phenotype appeared due to a deletion of one nucleotide in exon 3, causing a frameshift mutation in the messenger RNA and premature generation of a stop codon. The GPC and GPD protein sequences are therefore interrupted in the extracellular domain and probably intracellularly degraded.