The H blood group system consists of one antigen, H, that is carried on glycolipids and glycoproteins on the RBC membrane, where it is synthesised by the fucosyltransferase product of the FUT1 gene; as well as on glycoproteins on epithelial cells and in body fluids, where it is synthesised by the fucosyltransferase product of the FUT2 gene. In group O individuals, H antigen is the terminal antigen however, in group A and B individuals, the H antigen serves as the precursor structure for A and B blood-group-specific glycosyltransferases. Thus, group O people will test strongly H+ whereas groups A, B and AB will express very little H antigen. Mutations that negatively affect the a2FucT1 enzyme activity (encoded by FUT1) will result in reduced or absent H production (and a concomitant decrease in A and/or B antigens in individuals where those enzymes are encoded). Total absence of H, A and B antigens is called the Oh or Bombay phenotype. Weak expression is referred to as the paraBombay phenotype. The enzymes a2FucT1and a2FucT2 are single pass type II membrane glycoproteins in the Golgi. The a2FucT1 protein consists of 365 amino acids and is encoded by FUT1 or H, if analysis is to predict a blood group antigen. The FUT2 gene produces two transcripts; one of 343 amino acids and another more abundant form of 332 amino acids. The longer transcript encodes a protein with approximately one fourth the enzymatic activity and the shorter form is considered to be the active enzyme. Thus, the numbers given below are counted from the second initiating codon. The a2FucT2 protein is encoded by FUT2 or Se, if analysis is to predict a blood group antigen.