Folic acid (FA, folate or vitamin B9), is a vital nutrient required by all living cells for nucleotide biosynthesis and for the proper metabolic maintenance of 1-carbon pathways.[4] Aside from its cofactor role for intracellular enzymes, FA also displays high affinity for the folate receptor (FR), a glycosylphosphatidyinositol-linked protein that captures its ligands from the extracellular milieu and transports them inside the cell via a non-destructive, recycling endosomal pathway.[5][6] The FR is also a recognized tumor antigen/biomarker.[7][8][9] Because of this, diagnostic and therapeutic methods which exploit the FR's function are being developed for cancer.