FUT3 or the 'Lewis gene', codes for the Fucosyltransferase-3 enzyme, Galactoside-3(4)-L-fucosyltransferase, which adds fucose to precursor oligosaccharides to synthesize the Lewis antigen. The version of Lewis antigen formed is determined by FUT2. The active form(Le) is dominant over the inactive(le). Mutations is the gene cause non-function, resulting in the Lewis-negative type. Multiple variants are known.
The Lewis antigen is needed to express CA19-9, an important biomarker used to track tumor status, primarily pancreatic cancer. In 10% of Caucasians CA19-9 is not produced.
Le status is associated with infection susceptibility.