ESR1 and breast cancer: To explore the role of different forms of ESR1–CCDC170 fusions in endocrine resistance, we engineered four major fusion variants, E2-E6, E2-E7, E2-E8 and E2-E10, that join the exon 2 of ESR1 with the exon 6, 7, 8, or 10 of CCDC170, respectively, into the T47D luminal breast cancer cells known to be dependent on estrogen [26].