The Lewis antigen system is a human blood group system based upon genes on chromosome 19 p13.3 (FUT3 or Lewis gene) and 19q13.3, (FUT2 or secretor gene). Both genes are expressed in glandular epithelia and have dominant alleles (Le and Se, respectively) coding for enzymes with fucosyltransferase activity and recessive alleles (le and se, respectively) that are not functional.