MAP2K5 and restless legs syndrome: Since the heterozygous Map2k5 mutant mice already displayed reduction in TH+ dopaminergic cells significantly on postnatal 18 days in our study, it is plausible to think that the Map2k5–Erk5 pathway regulates the dopaminergic development in an earlier period and contributes to central nervous system disorders such as RLS/WED and PD.