Effect of DSS1 over-expression on proliferation of the breast cancer cell lines MCF7 and MDA-MB-231. Human DSS1 cDNA was introduced using an ecotropic retroviral vector with GFP plus IRES sequence. (A) DSS1 + GFP vector and GFP alone vector transfectants were sorted and used for cell proliferation assay on day 4 to day 5 after transfection. DSS1 + GFP and GFP alone transfectants showed similar proliferation rates in the MTT assay (left) and cell counting (right). (B) Drug sensitivity was examined in the MCF7 transfectants using an optimized dose of CPT (50 μM) for 2 to 3 days. Results of the cell cycle profile (left) are shown in the graph, which represents more than three independent experiments. The proportions of dead cells were measured by counting the sub-G1 fraction and compared in the graph (right). Statistical significance was calculated using Student’s t-test, with *P < 0.05 as significant. (C) Drug sensitivity was similarly compared in MDA-MB-231 cells. n.s.: not significant.