MAPK9 and breast cancer: 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.