The Rh blood group antigens D, Cc and Ee are encoded by two highly related genes, RHD and RHCE. Almost all red cells which carry D and all cells which carry C also express the G (Rh12) antigen. In this report we have determined the molecular basis of the DIIIb category phenotype which represents a very rare condition characterized by the presence of most of the D epitopes and the total absence of the antigen G. mRNA sequencing and Southern blot analysis of two unrelated samples indicated that the DIIIb category phenotype is associated with a segmental DNA exchange between exon 2 of the RHD and RHCE genes resulting in three D-->c amino acid substitutions (Ile60Leu, Ser68Asn and Ser103Pro).