It is well known that xenobiotics exert their biological effects via the alteration of protein expression, through up-and down-regulation, alteration of protein synthesis and degradation rates, or chemically modifying existing proteins. Either way, chemical toxicants can have very complicated cellular effects-those that can be studied by proteomics. Proteomics measures the quantitative and qualitative changes in cellular or tissue protein expression and explores protein-protein and protein-ligand interactions. Toxicoproteomics is an approach for the identification and characterization of proteins whose expression is altered by chemical exposure, and it is complementary to toxicogenomics.