The primary route for membrane transport of reduced folates into mammalian cells and tissues is the ubiquitously expressed reduced folate carrier (RFC). RFC is also involved in specialized tissue functions related to folates, including absorption across the intestinal epithelium and transplacental transport of folates. This chapter summarizes the current understanding of the major human RFC gene and transcript variants, best typified by G80A that results in a Arg to His substitution at position 27, a functional 61 bp deletion in promoter A, and a CATG insertion at position 191 that results in loss of functional carrier. The occurrence of RFC gene and transcript sequence variants might alter levels of tetrahydrofolate cofactor transport into cells and tissues at the level of modified or decreased RFC, resulting in effects on folate absorption, or downstream effects on folate-dependent biosynthetic pathways. These may contribute to inter-individual differences in susceptibilities to cardiovascular disease, fetal abnormalities, or cancer, particularly in combination with low serum folates. For patients with cancer, treated with antifolate chemotherapy, RFC variants may alter drug pharmacokinetics and antifolate uptake by both tumor and normal cells, thus influencing antitumor activities and toxicities associated with the administration of chemotherapy. Transport defects resulting from changes in RFC structure or expression may be compounded by changes in the catalytic activities of folate-dependent interconverting and biosynthetic enzymes (e.g., 5,10-methylene tetrahydrofolate reductase) that impact cellular distributions of individual tetrahydrofolate forms. By identifying and better understanding naturally occurring RFC gene and transcript variants, it may be possible to develop genetic screens to identify particular groups of patients who may be predisposed to pathologies resulting from folate deficiencies, or who may be subject to unacceptable toxicities or enhanced antitumor effects of antifolate therapeutics.