PLA2G6 and Parkinson disease: Four neurological disorders with infantile, juvenile, or early adult‐onset are associated with PLA2G6 genetic alterations, namely infantile neuroaxonal dystrophy (INAD), atypical neuroaxonal dystrophy (ANAD), dystonia‐parkinsonism (DP), and autosomal recessive early‐onset parkinsonism (AREP).