We are developing a chemical biology technique called amber suppression or unnatural amino acid mutagenesis in mammalian cells. Unnatural amino acids with a variety of chemical functionalities can be inserted site-specifically into a target protein within a living cells in response to a recoded stop codon. Such strategy for genetic code expansion holds great potential for engineering proteins in vivo, but its applicability to basic biology and human disease research has been hampered by the limited biological systems it could be established in. We have developed an optimized system that allows highly efficient amber suppression in mammalian cells (read more). Using this system, we aim to do biochemistry in a living cell, controlling and observing proteins ‘at work’ in their in vivo environment.