Recent reports have demonstrated that Jnk1, Jnk2 or composite Jnk1 and 2 deficiencies promote primary tumor formation in various mouse models of breast cancer [42], [43], [44] These data are consistent with our findings that Klf4, by repressing Jnk1 gene expression, prevents apoptosis and promotes primary tumor growth while inhibiting metastasis formation. Here, MAPK8 is linked to breast carcinoma.