As TOP1 poisoning by CPT has been shown to lead to micronuclei formation [11], we have determined the levels of micronuclei in human SCLC (H209, H889 and DMS114) and HeLa cells treated with low doses of Top1 poisons (CPT and LMP776) and released from drug treatment for 48 h (Fig. 1a). Here, TOP1 is linked to small cell lung carcinoma.