The human concentrative (Na+-linked) plasma membrane transport proteins hCNT1 and hCNT2, found primarily in specialized epithelia, are selective for pyrimidine nucleosides (system cit) and purine nucleosides (system cif), respectively. Both have orthologs in other mammalian species and belong to a gene family (CNT) that also includes members in lower vertebrates, insects, nematodes, pathogenic yeast and bacteria. The CNT transporter family also includes a newly identified human and mouse CNT3 transporter isoform. This paper reviews the studies of CNT transport proteins that led to the identification of hCNT3 and mCNT3, and gives an overview of the structural and functional properties of these latest CNT family members. hCNT3 and mCNT3 have primary structures that place them in a CNT subfamily separate from CNT1/2, transport a wide range of physiological pyrimidine and purine nucleosides and antineoplastic and antiviral nucleoside drugs (system cib), and exhibit a Na+:uridine coupling ratio of at least 2:1 (cf 1:1 for hCNT1/2). Cells and tissues containing hCNT3 transcripts include mammary gland, differentiated HL-60 cells, pancreas, bone marrow, trachea, liver, prostrate and regions of intestine, brain and heart. In HL-60 cells, hCNT3 is transcriptionally regulated by phorbol myristate (PMA). The hCNT3 gene, which contains an upstream PMA response element, mapped to 9q22.2 (cf chromosome 15 for hCNT1 and hCNT2).