ESR1 and neoplasm: We observed a tendency in biomarker conversion towards a more aggressive phenotype with significant downregulation of ESR1 and PGR in biopsies from all MT sites and in all molecular subtypes of MT, as well as a boost in the proliferation marker MKI67 in all phenotypes (p < 0.001) The discordance in mRNA biomarker assessment between PT and MT due to receptor conversion necessitates dynamic monitoring of tumor biomarkers, possibly via liquid biopsy, e.g. circulating tumor cells (CTCs), or rebiopsy of metastatic lesions.