ESR1 and cancer: Whole exome sequencing (WES) analysis of 30 PDXs (including 18 HER2+, 6 estrogen receptor-positive (ER+), and 6 triple-negative (TN) samples) revealed that genetic mutations and copy number variations found in major cancer genes are clustered in a number of main signaling pathways (Fig. 1a and Supplementary Fig. 1a).