ESR1 and neoplasm: Exact quantification of the underlying target gene and protein expression could essentially facilitate exploration of tumor biomarker conversion upon disease progression, and therefore the present study used a highly standardized real-time quantitative polymerase chain reaction (RT-qPCR) kit to compare the expression levels of the ESR1, PGR, ERBB2, and MKI67 genes in matched pairs of BC PT and MT samples.