IL21 and cancer: Interleukin 21 (IL21) is an immune agonist that is an attractive cancer immunotherapeutic (Ma et al., 2003; Ugai et al., 2003; Di Carlo et al., 2004; Sivakumar et al., 2004; Spolski and Leonard, 2008; Xu et al., 2015; Lewis et al., 2017; Deng et al., 2020).