Tyrosine kinase is an enzyme that phosphorylates the hydroxyl group of the side chain of tyrosine residues in proteins. When a protein kinase phosphorylates substrate proteins by transferring phosphate from ATP to a substrate protein, the three-dimensional structure of the substrate protein changes and its activity is altered. This change also modulates the cell function. Serine/threonine kinases have been known as typical intracellular protein kinases. However, since the oncogene product of retrovirus was found to work as a tyrosine kinase, various new tyrosine kinases have been reported. The tyrosine kinases are cell growth factor receptors and oncogene products, so they are the research targets for cell signaling, cell proliferation and carcinogenesis. TCI offers small molecule inhibitors for tyrosine kinases involved in cancer research.