A 52‐week postmarketing surveillance study was initiated to evaluate the safety and effectiveness of guselkumab, a human anti–interleukin 23 subunit p19 monoclonal antibody, in Japanese patients with psoriasis vulgaris, psoriatic arthritis, generalized pustular psoriasis, and erythrodermic psoriasis in real‐world practice. The gene discussed is IL23A; the disease is psoriatic arthritis.