Ideally, subjects at risk, such as carriers of specific mutations in the glucosylceramidase beta 1 (GBA1) or LRRK2 genes or subjects with prodromal symptoms such as rapid eye movement (REM) sleep without atonia or decreased sense of smell, could undergo a simple procedure to extract a skin biopsy, undergo full SRM aggregate analysis, and potentially be treated (if positively diagnosed with the pathological biomarker for PD) before irreversible neural damage has occurred. Here, GBA1 is linked to Parkinson disease.