[³H]5-methylTHF uptake at low pH in lymphocytes derived from patients P6 and P7 and their parents. Uptake of 0.5 μM [³H](6S)5-methylTHF at pH 5.5 was assessed over 2 minutes. F+/− and M+/− indicate cells derived from father and mother, respectively, who each carry 1 mutated PCFT allele. The data are the mean plus or minus SEM from 3 independent experiments.