Involved in the interconversion between alpha- and beta-L-fucoses. L-Fucose (6-deoxy-L-galactose) exists as alpha-L-fucose (29.5%) and beta-L-fucose (70.5%), the beta-form is metabolized through the salvage pathway. GDP-L-fucose formed either by the de novo or salvage pathways is transported into the endoplasmic reticulum, where it serves as a substrate for N- and O-glycosylations by fucosyltransferases. Fucosylated structures expressed on cell surfaces or secreted in biological fluids are believed to play a critical role in cell-cell adhesion and recognition processes. alpha-L-fucose = beta-L-fucose Carbohydrate metabolism; L-fucose metabolism. Mainly homodimer, but exists also as homotetramer, homooctamer, and homodecamer. The homodimeric form seems catalytically inactive. Widely expressed in various tissues and cell lines, including kidney, liver, and pancreas, marginally in muscle and testis. Deficient female mice show reduced sexual receptivity, as measured by lordosis quotient and masculinized sexual behaviors in female mice. Display a reduced number of tyrosine hydroxylase-positive neurons in the anteroventral periventricular nucleus, a sexually dimorphic feature in rats and mice. Embryos at 16.5 dpc have reduced alpha-feto-protein levels and reduced fucosylation of alpha-feto-protein. Belongs to the RbsD / FucU family. Truncated N-terminus.