The Rh blood group antigens are encoded by two highly related genes, RHD and RHCE, and the sequence of the common alleles (D, Ce, CE, ce, and cE) of these genes has been previously elucidated. In this report, Rh transcripts and gene fragments have been amplified using polymerase chain reaction from the blood of donors with the CW+ andCX+ phenotypes. Sequence analysis indicated that the expression of the CW (Rh8) and CX (Rh9) antigens are associated with point mutations in the RHCE gene, which provides the definitive evidence that the CW and CX specificities are encoded by the same gene as the Cc and Ee antigens. As compared with the common (CW- and CX-) transcripts of the RHCE gene, the CW+ and CX+ cDNAs exhibited A122G and G106A transitions that resulted in Gln41Arg and Ala36Thr amino acid substitutions in the CW+ and CX+ polypeptides, respectively. Therefore, although the CW and CX specificities behave serologically as if they were allelic, they cannot not be considered, stricto sensu, as the products of antithetical allelic forms of the RHCE gene. Based on the CW-/CW+ nucleotide polymorphism, a polymerase chain reaction assay useful for diagnosis purposes has been developed that detects the presence of the CW+ allele by the use of an allele-specific primer.