HSP90 has been described as a key player to stabilize proteins that are particularly important in cancer cells including BCR-ABL (BCR, breakpoint cluster region; ABL, Abelson), ERB-B2 (erythroblastic oncogene B), proto-oncogene B-Raf (BRAF), AKT Serine/Threonine Kinase (AKT), vascular endothelial growth factor receptor (VEGFR), FMS like tyrosine kinase 3 (FLT3), androgen and estrogen receptors, hypoxia-inducible factor (HIF-1α) and a constantly growing list, affecting a variety of cancer-related functions (Welch and Feramisco, 1982; Powers and Workman, 2006). This evidence concerns the gene ESR1 and cancer.