For example, in a single CTC analysis study of advanced estrogen-receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer, 84% of patients had CTCs expressing HER2-positive and cultured HER2+, and HER2− CTCs spontaneously interconverted their phenotype according to flow cytometry identification [44]. The gene discussed is ESR1; the disease is breast cancer.