During studying effects of the Hsp90 inhibitors (17AAG, NVP-AUY922, geldanamycin and radicicol) on the cellular radiation response, we found that 35–500 nM of the drugs significantly increased the radiosensitivity of MCF-7, HeLa, KTC-1, PC-3, Myc-CaP, HT 1080 and A549 cancer cell lines as well as actively proliferating endothelial cells, i.e. only those cell cultures in which the prominent Hsp70 induction was found after the inhibitory treatment. This evidence concerns the gene MYC and cancer.