Because disruption of ephrin A/EphA signaling reduced dopaminergic innervation in the nigrostriatal system21 and ephrin A5 signaling is implicated in accurate projection of dopaminergic axons from the substantia nigra into the striatum,22 it is assumed that ephrin A2 similarly regulates the dopaminergic axon guidance, thus its loss of functions may also be associated with PD. This evidence concerns the gene EFNA2 and Parkinson disease.