The structural and mechanistic insight into the modulation of IL-21 function CHENZHIAN 2017 Clinical trials to target interleukin-21 (IL-21) have achieved promising results to treat cancers and infections. Modulating the bioactivity of this cytokines is, therefore, an approach to deliver a therapeutic efficacy. By crystallography methodology, I revealed the mechanism of an agonistic antibody to human IL-21, which significantly enhances the bio-activity of IL-21 ex vivo and in vivo. Also, based on this structural information, I identified a previously unknown IL-21 binding partner, which could regulate the function of IL-21 physiologically.