ESR1 and cancer: Besides ER expression, better efficacy in LR-BT474 cells might be from the fact BT474 may have more deregulated targets for HSP90 inhibitor due to mutated PIK3CA. Although previous clinical studies did not include precise genomic information, the data presented here partially explain the significant gap with ganetespib between promising pre-clinical activity [43] and modest clinical efficacy in metastatic HER2 (+) cancers [31].