MAPK8 and Parkinson disease: On the other hand, phosphorylated p38 protein levels, normalized by p38, were not significantly elevated in PD patient-derived cortical neurons (Fig. 4E, F, Additional file 1: Fig. S3G–I), suggesting that among the different MAPK cascades, the ERK1/2 and JNK cascades are activated in PD patient-derived cortical neurons.