FNDC5 and myotonic dystrophy type 1: Plasma irisin levels in DM1 patients ranged 0.68 to 2.52 ng/mL; median plasma irisin level in DM1 patients was 1.4 (1.1–1.5) ng/mL, and it was significantly lower than the median level detected in healthy controls (P < 0.0001) (Figure 1A).