ESR1 and breast carcinoma: Samples were obtained from the following studies: IMPACT‐CRC study on colon cancer (ClinicalTrials.gov, number NCT02117466) [10], REGORA study on colon cancer (ClinicalTrials.gov, number NCT02800330), START‐TKI study on lung cancer (CCMO number, NL58664.078.16) [11], TAX‐ESR1 study on breast cancer (trialregister.nl, number NL7280), and the CareMore‐Trastuzumab study on breast cancer (trialregister.nl, number NL4977).