SEM1 and breast cancer: DSS1 over-expression renders breast cancer cells more resistant to treatment with CPT, an inhibitor of type 1 topoisomerase (which cleaves one strand of double-stranded DNA), compared with the GFP-control MCF7 transfectants (Figure 2B), while no change was detected in DSS1 over-expressing MDA-MB-231 transfectants (Figure 2C).