Tyrosine kinases are a group of chemical messengers (enzymes) produced by body cells. They are part of the cell signalling system that tells cells when to divide and grow. Genes that control tyrosine kinase can be abnormal in cancer cells. The abnormal TK then sends too many signals to the cancer cell telling it to grow. Drugs that block tyrosine kinase are called tyrosine kinase inhibitors or TKIs. They can help to stop cancers from growing, for example imatinib (Glivec) and erlotinib (Tarceva).