This review describes the current state of knowledge of the Raph blood group system, which consists of a single antigen, MER2. MER2 was initially classified as a high-incidence antigen in the 901 series of blood groups, formerly known as 901011, but was reclassified as an antigen in the Raph blood group system in 2004. There have been six reports of human alloantibodies to MER2. Three of the subjects were found to have a stop codon in the