Previous studies with geldanamycin derivatives have been valuable as a proof of concept, showing that inhibition of HSP90 has anti-cancer and radiosensitizing properties in several tumor-derived cell lines in vitro (including prostate, lung, colorectal, glioma, and pancreatic carcinomas) and in vivo through tumor xenograft models (human cervical, prostate and head and neck squamous cell carcinoma) [120–126]. This evidence concerns the gene HSP90AB1 and cancer.