The assay was performed as described previously.16 In brief, cells were seeded at 300–500 cells per well in 6-well plates containing complete DMEM/F-12 on day 0 and incubated at 37°C and 5% CO2 for 10 days. On day 10, cells were fixed with 4% polyformaldehyde for 15 min and stained with 1% crystal violet before quantification. The experiments were triplicated, and the numbers of colonies containing more than 50 cells were microscopically counted.