[³H]Folic acid uptake in cells stably transfected with the WT and mutant PCFT. To assess the functionality of the mutant R113C PCFT, [³H]folic acid influx into stable transfectants overexpressing the WT and mutant R113C PCFTs was determined as detailed in Document S1. Note the statistically significant, marked increase in folic acid uptake in WT PCFT transfectants but not in mutant R113C or mock transfectants. Data depicted are means plus or minus SD from 3 independent experiments performed on separate days.