Immunohistochemical (IHC) evaluation of skin biopsy samples, as demonstrated in Fig. 1a, revealed that CD8/CD4 ratios of epidermal-infiltrating mononuclear cells were significantly increased in cases of anti-PD-1 antibody-induced psoriasis-like dermatitis (median ± standard deviation [SD], 3.48 ± 1.0) compared to that in cases of idiopathic psoriasis (1.06 ± 0.19, P = 0.008 by Mann–Whiney U test, Fig. 1b). This evidence concerns the gene PDCD1 and dermatitis.