In a phase II clinical trial (PALOMA-1), the addition of palbociclib to letrozole significantly increased the median progression-free survival (PFS) from 10.2 months to 20.2 months [15] vs. letrozole alone for post-menopausal women with previously untreated ESR1+/HER2− advanced breast cancer. The gene discussed is ESR1; the disease is breast cancer.