In addition, a positive correlation was observed between increased serum irisin levels and improved BBS scores of PD patients (R = 0.49, p = 0.018, Supplementary Fig. 1e), whereas increased serum irisin levels had no association with improved UPDRSIII scores (R = 0.−0.087, p = 0.69, Supplementary Fig. 1d). Here, FNDC5 is linked to Parkinson disease.