In this study, we microscopically observed a remarkably high density of the cancer cells in the orthotopic tumors, which showed a high expression level of CXCR4, in contrast to the cancer cells in the micro-metastatic lesions in the lung that showed low cell numbers in the tissues and a low expression level of CXCR4 (Figs 1 and 3, S2B and S3 Figs). The gene discussed is CXCR4; the disease is cancer.