ESR1 and breast cancer: We extended our analysis of p16INK4A expression to include HER2-negative subtypes and found that across breast cancer subtypes (ER+ HER2−, HER2+ and ER− HER2−), p16INK4A expression is well correlated (r = 0.7, p = 1.6 × 10−5) between PDX and patient tissues (Supplementary Fig. 2b, c).