CEACAM1 and neoplasm: Another heavily studied inhibitory checkpoint mediating T cell exhaustion is the T-cell immunoglobulin and mucin-domain-containing molecule 3 (TIM-3), which is expressed on numerous types of immune cells.28 TIM-3 also has numerous known ligands; binding to galectin 9 leads to T cell death, while CEACAM1 binding appears to promote tolerance, although paradoxically CEACAM1-deficient animal models show increased tumor growth.