HumanbloodgroupBgalactosyltransferase (GTB) catalyzes the galactosylation of the H antigen and is responsible for the formation of the bloodgroup antigen of phenotype B. The ABO bloodgroup system is well studied and routinely serotyped before transfusion and transplantation. Blood type subgroups have been repeatedly linked to an increased occurrence of diseases (e.g., a highly increased incidence