Among selection criteria used, age at onset under 55 years proved to perform better in predicting patients’ genetic status in monogenic PD: 86% of patients with a positive genetic test, in fact, had disease onset under 55 years vs 62% of negative subjects (OR 3.8, 95% IC 1.5–9.3; p 0.0037); 25.8% of subjects with symptom onset ≤ 55 years received a genetic diagnosis vs 8.5% of those with later onset, who only had mutations in GBA1 (4 subjects) and LRRK2 (2 patients); 1 subject with pathogenic biallelic PRKN mutations had symptoms onset at 55 years. The gene discussed is GBA1; the disease is Parkinson disease.