Researchers have discovered that an enigmatic type of T cell in the liver can be effective at attacking liver cancer cells, but its anti-tumor activity is suppressed in some patients. At the same time, the investigators have shown how treatment with immune checkpoint inhibitors can reverse this effect, helping this type of immune cell, called a mucosal-associated invariant T (MAIT) cell, better attack and infiltrate liver tumors. The finding could explain why some patients with liver cancer, but not others, respond to treatment with immunotherapy.