The primary objective is to assess the accuracy and feasibility of a novel multi-gene target panel NGS assay on circulating tumor DNA (ctDNA) to detect molecular alterations of AKT1, ERBB2, ESR1, KRAS, PIK3CA, and TP53 genes in women with BC undergoing HT plus CDK4/6i treatment. This evidence concerns the gene ESR1 and neoplasm.