MTT assay

The lentivirus-infected osteosarcoma U2OS cells were seeded into 96-well tissue culture plates at a density of 2×103 cells/well and incubated at 37°C for between 1 and 5 days. Each day, 20 µl MTT solution (Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) was added to each well. Following 4 h of incubation at 37°C, the medium was discarded and 100 µl DMSO was added to each well to dissolve the MTT crystals. Optical density was measured during growth at a wavelength of 595 nm using a microplate reader.