Interleukin 23 (IL-23) is a heterodimeric pro-inflammatory cytokine that shares traits with IL-12. Both cytokines contain the p40 subunit which binds to the receptor chain IL-12Rβ1. However, the two cytokines exert distinct non-redundant biological functions. IL-23 has been implicated as an inflammation mediator in several autoimmune diseases, and has also been found to promote tumor growth. Therapeutic agents targeting both IL-12 and IL-23 cytokines are currently used to treat psoriasis and psoriatic arthritis, and related agents are in clinical testing for a variety of inflammatory disorders.