Plasma membrane transporter mediating the uptake by cells of the water soluble vitamin B2/riboflavin that plays a key role in biochemical oxidation-reduction reactions of the carbohydrate, lipid, and amino acid metabolism. riboflavin(in) = riboflavin(out) Predominantly expressed in small intestine. Highly expressed in jejunum and ileum. Also expressed in testis and at lower level in lung, kidney, stomach and colon. Belongs to the riboflavin transporter family.