MAPK9 and cardiac arrhythmia: The inter-relationship between ROS and JNK2 in CaMKIIδ activation remains to be determined. Overall, hyperactivation of CaMKIIδ under a stressed and/or diseased state leads to maladaptive cardiac remodeling including channel dysfunction, impaired Ca2+ homeostasis, and contractile dysfunction resulting in deteriorated cardiac function and increased risk of arrhythmias as summarized in Fig. 1.