While low doses of l-DOPA do not cause cytotoxicity in PC12 cells [9] and are beneficial for the treatment of PD by increasing dopamine levels in the brain [21], repeated treatments with l-DOPA at non-toxic levels can cause sustained ERK1/2 and JNK1/2 activation in PC12 cells, which in turn induce c-Jun phosphorylation at Ser63 and c-Jun expression and result in cell death [22]. Here, MAPK3 is linked to Parkinson disease.