Bivariate correlation analysis suggested serum IL-17A significantly correlated with Claudin-1 only in subtypes of late-onset psoriasis (r = 0.630, p = 0.028), but not of early-onset psoriasis (r = − 0.196, p = 0.300), mild psoriasis (r = − 0.120, p = 0.647), and moderate-to-severe psoriasis (r = − 0.041, p = 0.844), as shown in Table S4. Here, CLDN1 is linked to psoriasis.