ESR1 mutation analysis
In the trial, plasma was collected from consenting patients at baseline and on day 1 of cycle 2. DNA was isolated from the total amount of plasma that was available, being a median of 1440 uL (range: 60-4000 uL), using the QIAamp Circulating Nucleic Acids kit (Qiagen, Venlo, the Netherlands). Subsequently, the presence of 