Thus, we considered that expression levels of Per2 and Bik could also be a critical biomarker for chronotherapy of MTX not only for RA, but also for other diseases such as acute lymphocytic leukemia, non-Hodgkin’s lymphoma, osteosarcoma, and breast cancer, in accordance with a previous report that circadian clock genes could be a biomarker for chronotherapy [42]. The gene discussed is PER2; the disease is breast cancer.