ESR1 and breast cancer: SSEA1 correlates with the invasive phenotype in BCa; its enhanced expression in estrogen receptor alpha (ERα)-negative tumors was observed and was also reported as a mediator of adhesion between breast carcinoma cells and activated endothelium (14, 15) This stem cell marker has been observed overexpressed in circulating tumor cells present in the cerebrospinal fluid of breast cancer patients with leptomeningeal metastasis and increased levels of its sialylated form, CD15s, were proposed as circulating tumor biomarker of breast cancer metastasis (16).