Nucleoside selectivity of hCNT3 and mutants. Uptake of 20 μm radiolabeled uridine (U), adenosine (A), guanosine (G), thymidine (T), cytidine (C), and inosine (I) was measured in oocytes producing E343Q (A), E343D (B), E343C (C), E519D (D), E519Q (E), or hCNT3 (F) in 100 mm NaCl, pH 7.5 (black bars), or 100 mm ChCl, pH 5.5 (open bars). Values were corrected for basal nonmediated uptake in control water-injected oocytes and are means ± S.E. of 10–12 oocytes.