TK1 and colorectal cancer: In a colorectal cancer cell line, loss of TK1 expression confers resistance to trifluridine, but retains full sensitivity to 5-FU [34], even though TK1 is critical for phosphorylation of the 5-FU-derivative 2′-deoxy-5-fluorouridine (5-FdU) [303] which–however–can be circumvented by RNR-mediated reduction of 5-FU diphosphate (5-FUDP) to 5-FdUDP.