A diet rich in the amino acid arginine might help the immune system fend off cancer and infection. The researchers tinkered with arginine levels in mouse and human cells in the lab. Cutting back on arginine reduced levels of key immune proteins, the team found. They display snippets of viral or cancer proteins, like a police officer holding up a sketch of a criminal. When the researchers fed mice at risk for colon cancer a low arginine diet, MHC-1 levels went down, and the number of colon tumors went up.