Primer extension by compounds III–IV. Lane 1, primer + TDTP+ (control); lane 2, as in lane 1 + 10 µM dTTP; lanes 3–5, as in lane 2 + 50, 100 and 300 µM IIIb, respectively; lanes 6–8, as in lane 2 + 50, 100 and 300 µM IVa, respectively; lanes 9–11, as in lane 2 + 50, 100 and 300 µM IIIc, respectively; lanes 12–14, as in lane 2 + 50, 100 and 300 µM IVb, respectively.