(A) In vitro chemosensitivity to 5-fluorouracil (5-FU) plus leucovorin (LV) was significantly different among HCT116 colon cancer cells expressing the sense (FPGS-S), endogenous (vector alone (VA)), and antisense (FPGS-AS) folylpolyglutamate synthetase (FPGS) (p<0.001). Chemosensitivity of cells expressing the sense FPGS to 5-FU+LV was significantly enhanced compared with those expressing endogenous or the antisense FPGS (p<0.0001). In contrast, chemosensitivity of cells expressing the antisense FPGS was significantly decreased compared with those expressing endogenous or the sense FPGS (p = 0.0122 and p<0.0001, respectively). (B) In vitro chemosenstivity to methotrexate (MTX) was not significantly different among HCT116 cells expressing the sense, endogenous, and antisense FPGS (p = 0.548).