Of note, 24 of these patients had P/LP variants in GCH1, a gene classified as a dystonia–parkinsonism gene based on the recommendations of the International Parkinson and Movement Disorder Society Task Force.1 Although GCH1 variants have been described to date in numerous classical PD patients, making GCH1 an excellent PD-related gene candidate, our internal gene–disease validity assessment under the ClinGen framework28 showed only a ‘moderate relationship’ between GCH1 and AD PD. The gene discussed is GCH1; the disease is Alzheimer disease.