In order to examine a potential role of DOT1L in the DNA damage response to DNA double-strand breaks (DSB), initially DOT1L was efficiently depleted in U2OS osteosarcoma cells, a cell line widely used to study DNA repair mechanisms, and DSBs were induced by the radiomimetic drug neocarzinostatin (NCS). The gene discussed is DOT1L; the disease is osteosarcoma.