ESR1 and breast cancer: ADAM12 mRNA is alternatively spliced, and high levels of ADAM12 transcript variant 1 (encoding the transmembrane protein isoform ADAM12-L) are associated with poor prognosis and decreased metastasis-free survival times in estrogen receptor (ER)-negative, progesterone receptor (PR)-negative, and human epidermal growth factor receptor 2 (HER2)-negative (triple-negative) early stage breast cancers without systemic treatment, but not in HER2-positive or ER-positive tumors [15, 16].