Argininosuccinate synthetase (ASS1) expression differs between cancers, being silenced in a number of auxotrophic tumors (Ensor et al., 2002; Fiedler et al., 2015; Kobayashi et al., 2010; Liu et al., 2017; Nicholson et al., 2009; Ohshima et al., 2017; Syed et al., 2013; Szlosarek et al., 2017; Werner et al., 2019) but expressed in other cancers (Delage et al., 2010; Henriet et al., 2017; Rho et al., 2008; Szlosarek et al., 2007; Tsai et al., 2018). The gene discussed is ASS1; the disease is cancer.