CD226 and neoplasm: Although less well studied, CD226 is a costimulatory receptor regulated by TIGIT and exhibits multifaceted functions in anti-tumor CD8+ T cell responses (Gilfillan et al., 2008; Manes and Pober, 2011; Ralston et al., 2004; Shibuya et al., 1999, 2003; Shirakawa et al., 2005).