Glycinamide ribonucleotide transformylase (GAR Tfase) is a folate-dependent enzyme in the de novo purine biosynthesis pathway, which has long been considered a potential target for development of anti-neoplastic therapeutics. Here we report the biological and X-ray crystallographic evaluations of both independent C10 diastereomers 10S and 10R methylthio-DDACTHF bound to human GAR Tfase, including the highest resolution apo GAR Tfase structure to date (1.52 Å). Both diastereomers are potent inhibitors (10R Ki = 210 nM, 10S Ki = 180 nM) of GAR Tfase and exhibit effective inhibition of human leukemia cell growth (IC50 = 80 and 50 nM, respectively). Their inhibitory activity was surprisingly high and these lipophilic C10-substituted analogs show distinct advantages over their hydrophilic counterparts, most strikingly in retaining potency in mutant human leukemia cell lines that lack reduced folate carrier protein activity (IC50 = 70 and 60 nM, respectively). Structural characterization reveals a new binding mode for these diasterisomers, in which the lipophilic thiomethyl groups penetrate deeper into a hydrophobic pocket within the folate-binding site. In silico docking simulations of three other sulfur-containing folate analogs also indicates that this hydrophobic cleft represents a favorable region for binding lipophilic substituents. Overall, these results suggest sulfur and its substitutions play an important role in not only the binding of anti-folates to GAR Tfase, but also in selectivity and cellular activity (growth inhibition), thereby presenting new possibilities for future design of potent and selective anti-folate drugs that target GAR Tfase.