Even those of us whose whose biochemistry is a little shaky are likely to know that there are twenty amino acids that form the building blocks of proteins in life on earth. Now even that simple factoid is no longer absolutely true. A group of scientists headed by Peter G. Schultz of the Scripps Research Institute have created a new form of E. coli that uses (and makes) p-aminophenylalanine as well as the usual twenty. This feat was accomplished by changing the organism's DNA and RNA. The trick of making organisms produce non-natural proteins had been done many times before, but this is the first instance in which a modified organism has also been made to incorporate the new amino acid into its own structure. Now the group will be trying to determine why life didn't do this before; why did nature stop with twenty amino acids, and why this particular twenty? See also the digest, "Reinventing Biology" by Stu Borman in Chemical and Engineering News 2003 81(3) [January 20, 2003] p. 7.