When the entire intent to treat population from both cohorts was analyzed together, PFS increased from 10.2 month to 20.2 months (HR 0.488, 95% CI 0.319–0.748; one-sided p = 0.0004) confirming the preclinical observations that addition of palbociclib to letrozole would improve efficacy in ER+/HER2− breast cancers. Here, ESR1 is linked to breast cancer.