Furthermore, CTCs isolated by the CellSearch System targeted against markers such as estrogen receptor, B-cell lymphoma 2 (BCL-2), Human Epidermal Growth Factor Receptor (EGFR) 2, and Ki67 are crucial in the development of novel CTC-Endocrine Therapy Index, a parameter predicting response to endocrine therapy in patients with BC [28]. The gene discussed is ESR1; the disease is breast cancer.