In a randomized phase III study (PALOMA-3), in which the CDK4/6 inhibitors palbociclib and fulvestrant were tested in a group of women (521 patients) with advanced BC (estrogen receptor-positive BC and HER2-negative BC), there were changes in PIK3CA ctDNA levels in plasma samples after 15 days of treatment with palbociclib and fulvestrant. This evidence concerns the gene ESR1 and breast cancer.