While it is well recognized that riboflavin accumulates in breast milk as an essential vitamin for neonates， transport mechanisms for its milk excretion are not well characterized. The multidrug efflux transporter ABCG2 in the apical membrane of milk-producing mammary epithelial cells (MECs) is involved with riboflavin excretion. However， it is not clear whether MECs possess other riboflavin transport systems， which may facilitate its basolateral uptake into MECs. We report here that transcripts encoding the second (SLC52A2) and third (SLC52A3) member of the recently discovered family of SLC52A riboflavin uptake transporters are expressed in milk fat globules from human breast milk. Furthermore， Slc52a2 and Slc52a3 mRNA are upregulated in the mouse mammary gland during lactation. Importantly， the induction ofSlc52a2， which was the major Slc52a riboflavin transporter in the lactating mammary gland， was also observed at the protein level. Subcellular localization studies showed that green fluorescent protein-tagged mouse SLC52A2 mainly localized to the cell membrane， with no preferential distribution to the apical or basolateral membrane in polarized kidney MDCK cells. These results strongly implicate a potential role for SLC52A2 in riboflavin uptake by milk-producing MECs， a critical step in the transfer of riboflavin into breast milk.