Insertional inactivation of MALAT1 in MMTV-PyMT mouse which is a transgenic model of metastatic breast cancer, induced significantly increased lung metastasis of breast cancer cells (Kim et al., 2018), while insertional inactivation of MALAT1 in mouse with normal physiological condition showed no apparent phenotype (Nakagawa et al., 2012), suggesting MALAT1 is dispensable for development but plays important roles in suppressing breast cancer metastasis. The gene discussed is MALAT1; the disease is breast cancer.