TK1 and neoplasm: The tracer 3′-deoxy-3′-[18F]-fluorothymidine ([18F]FLT) has often been used for tumor imaging over the last years since it is a substrate of thymidine kinase-1 (TK1) that is overexpressed in proliferating cells during the late G1 and S phases of the cell cycle when the cell synthesizes DNA [18, 19].