The results observed in vitro using two widely recognized SS18:SSX fusion gene positive synovial sarcoma cell lines indicate not only an additive, but a synergistic anti-tumor activity of the Topo II inhibitors doxorubicin and etoposide when combined with peposertib (Figure 1 and Supplementary Figure S1). The gene discussed is SSX2; the disease is synovial sarcoma.