In our study, our inclusion criteria were: postmenopausal women with breast cancer (not less than 50 years old) who had progressive disease, and who were receiving sequential treatments with AIs or Selective estrogen receptor modulators (SERMs) (the university hospital medical information network (UMIN) Clinical Trials Registry (UMIN000002831) http://www.umin.ac.jp/ctr/index.htm). The gene discussed is ESR1; the disease is breast cancer.