GDP-L-fucose synthetase (FX protein), also designated Tissue-specific transplantation antigen P35B or GDP-keto-6-deoxymannose 3,5-epimerase, 4-reductase, belongs to the fucose synthetase family. FX is a red cell NADP(H)- binding protein that is important in leukocyte adhesion and trafficking processes. The FX protein, together with GDP-mannose 4,6-dehydratase, can convert GDP-mannose to GDP-L-fucose by catalyzing the two-step epimerase and reductase reactions. GDP-L-fucose is the substrate of several fucosyltransferases that function in the expression of many glycoconjugates such as blood group ABH antigens and developmental adhesion antigens. Defects in the gene encoding for the FX protein cause leukocyte adhesion deficiency (LAD).