This dichotomy of JNK functions is particularly evident in the context of breast tissue, where in vivo JNK1/JNK2 knockout models have demonstrated that JNK plays essential roles in maintaining the architecture of normal breast tissue [8], driving the genetic programs required for mammary gland involution post lactation [9] and preventing early tumor initiation events [10,11]. Here, MAPK9 is linked to neoplasm.