HSP90 inhibition impairs the survival of primary KRAS mutant colon tumor-initiating cells. (a) Characteristics of primary human colon cancer samples used for the generation of sphere cultures. (b) Photographs of colon cancer sphere cultures treated with 0.5 µM PU-H71 or PBS for 48 h. Bar, 200 µm. (c) Viability and proliferation of colon cancer sphere cultures incubated with 0.5 µM PU-H71 for 48 h. 6–12 independent experiments were done. Error bars represent mean ± SD. (d) Cumulative analysis of the response to PU-H71 of the different patient-derived sphere cultures according to KRAS status. *, P < 0.05; **, P < 0.01; ***, P < 0.0001.