ESR1 and breast cancer: For women with ER+ stage IV breast cancer treated with the combination of palbociclib (CDK4/6 inhibitor) plus letrozole, the median PFS was 20.2 months, a statistically significant improvement compared to the 10.2 months of PFS in women who received letrozole alone (HR = 0.488 [95 % CI: 0.32, 0.75]; p < 0.001 [6].