Mutations in LRRK2 are a major cause of inherited and sporadic forms of Parkinson's disease.39 We and our colleagues recently showed that LRRK2 is localized within the endosomal compartment and that it negatively regulates Rac1 activity in mammalian cell lines and the fly eyes.17 Several studies have shown the interaction between LRRK2 and signaling components of p38 MAPK and JNK pathways, but the specific links between them are lacking.40, 41, 42 Here we found that dLRRK kinase activity acts as a negative regulator of Egr signaling that lies upstream of Rac1 and Rab21. Here, MAPK8 is linked to Parkinson disease.