Serum APN/CD13 concentrations were significantly higher in patients with PC than in controls (median 44.6 U/mL, interquartile range (IQR) 22.4–59.3 vs. median 9.9 U/mL, IQR 7.4–13.7; P<0.001; Figure 1, Supplementary Table S2), and these values did not significantly differ among the three control groups (Figure 1). The gene discussed is ANPEP; the disease is pachyonychia congenita.