The expression of OR10A4, OR9A2, GRM2, and IFIT1B did not show any dependency on the H&Y scale (score ranges: 0, 1–2.5, or 3–5) (Supplementary Figure S3A–D) or UPDRS scores (Supplementary Figure S4A–D), indicating that these markers are poor indicators of PD severity. Here, GRM2 is linked to Parkinson disease.