PLA2G6 and neurodegenerative disease: The four most frequent subtypes have gene mutations in either; PANK2 [pantothenate kinase-associated neurodegeneration (PKAN)], PLA2G6 [PLA2G6-associated neurodegeneration (PLAN)], C19orf12 [mitochondrial-membrane protein-associated neurodegeneration (MPAN)], or WDR45 [beta-propeller protein-associated neurodegeneration (BPAN)].