The H antigen is formed with the addition of a fucose (Fuc) sugar onto one of two precursor oligosaccharide sequences, Type 1 (RBCs) or Type 2 (secreted) chains. The FUT1 gene (aka H gene) found in hematopoietic cells produces galactoside 2-α-L-fucosyltransferase 1 (FUT1 aka α-1,2-fucosyltransferase 1) which mediates the transfer of a fucose (Fuc) sugar to the galactose (Gal) sugar of the Type 2 chain precursor Gal-β1,4-GlcNAc-β1,3-Gal-R (where R is a glycosphingolipid) to form the H antigen (Larsen et al. 1990). This is an essential step for subsequent formation of A and B antigens. Mutations that inactivate the FUT1 gene can result in the 'Bombay phenotype' where no A, B or H antigens are produced on RBCs (Koda et al 1997, Kaneko et al. 1997).