In the open-label PADA-1 phase 3 trial, patients with HR+/HER2– breast cancer receiving palbociclib7 + AI were monitored for rising levels of ESR1 mutations using circulating tumor DNA (ctDNA) and were randomized to either continue with their current treatment or switch to palbociclib (same dose) + fulvestrant. This evidence concerns the gene ERBB2 and breast cancer.