qRT-PCR analysis revealed that unstimulated ear skin from PD-1−/− and WT mice contain similar low levels of psoriasis-related cytokines, IL-6, IL23-A, and IL-17A (P = 0.95, P = 0.57 and P = 0.21 by Mann–Whitney U test, respectively, Fig. 2g). Here, IL17A is linked to psoriasis.