PROK2 and REM sleep behavior disorder: In a subgroup of subjects, results were validated in serum.<h4>Results</h4>In the idiopathic rapid eye movement sleep behavior disorder group, prokineticin-2 protein expression was reduced in both ONs (immunofluorescence: F(1,26) = 15.289, p < .001; western blot: F(1,12) = 9.073, p = .011) and serum compared with HCs (western blot: F(1,12) = 4.557, p = .050).