[³H]5-methylTHF uptake in HeLa cells transiently transfected with the cDNA of PCFT mutants. Uptake of 0.5 μM [³H](6S)5-methylTHF was assessed at pH 5.5 and 37°C over 2 minutes; P values reflect differences in activities of the mutated carriers as compared with the mock transfectants. The data are the mean plus or minus SEM from 4 independent experiments.