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 (PubMed:20463145, PubMed:22864630, PubMed:23243084, PubMed:24253200, PubMed:27702554). Humans are unable to synthesize vitamin B2/riboflavin and must obtain it via intestinal absorption (PubMed:20463145). May also act as a receptor for 4-hydroxybutyrate (Probable). (Microbial infection) In case of infection by retroviruses, acts as a cell receptor to retroviral envelopes similar to the porcine endogenous retrovirus (PERV-A). riboflavin(in) = riboflavin(out) Riboflavin transport is Na(+)-independent but moderately pH-sensitive (PubMed:20463145). Activity is strongly inhibited by riboflavin analogs, such as lumiflavin (PubMed:20463145). Weakly inhibited by flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) (PubMed:20463145). Highly expressed in brain, fetal brain and salivary gland. Weakly expressed in other tissues. The disease is caused by variants affecting the gene represented in this entry. Belongs to the riboflavin transporter family.