Due to the lack of mouse models with which to reproducibly monitor spontaneous breast cancer metastasis, we ectopically expressed Id2 or Aldh3a1 in mouse 4T1-Luc cells and in the human breast cancer cell line, MDA-MB-231-Luc (Fig. 3a), and performed intracardiac inoculation of the manipulated cells into BALB/c and NSG mice, respectively. Here, ID2 is linked to breast cancer.