To determine if HUNK regulates breast cancer metastasis in vivo, we used the 4T1 cell line, which was originally isolated as subpopulation 410.4 derived from a spontaneously arising mammary tumor in BALB/cfC3H mice, and is defined as an animal model for stage IV human breast cancer with spontaneous metastatic capability [37–40]. The gene discussed is HUNK; the disease is breast carcinoma.