ITSN1 and Parkinson disease: In conclusion, we report a novel gene associated with PD and suggest that the loss of ITSN1 function may be involved in the pathogenesis of PD in at least one of three ways; (1) Inactive CDC42 and its downstream pathway results in degeneration of dopaminergic neurons, (2) inactive CDC42 cannot regulate the vesicle exocytosis of α-synuclein, and (3) disrupted synaptic vesicle transport, a known feature in the pathogenesis of PD1, via clathrin-mediated endo- and exocytosis.