Guselkumab is an antibody designed to bind to the p19 subunit of interleukin-23 (IL-23). Detailed studies of affected skin in patients with psoriasis demonstrated that guselkumab induced significant changes in the cellular, cytokine and gene expression profiles of psoriatic lesions accompanied by significant changes in clinical measures of disease activity. The significant clinical response in psoriasis was demonstrated in these studies by significant improvement as reflected in the percentage of patients achieving a Physicians Global Assessment score of 0 or 1, and a 90% or 100% improvement in the Psoriasis Area and Severity Index (PASI 90, PASI 100), as well as changes in quality of life measures. These changes were accompanied by a low frequency of adverse effects. The combination of efficacy and safety led to its approval by the Food and Drug Administration (FDA) for use in patients with moderate to severe psoriasis.