Proteins account for the catalytic and structural versatility displayed by all cells, yet they are assembled from a set of only 20 common amino acids. With few exceptions, only 61 nucleotide triplets also direct incorporation of these amino acids. Endeavors to expand the genetic code recently progressed to nucleus‐containing cells, after Chin et al.1 transferred Escherichia coli genes for a mutant tyrosine‐adaptor molecule and its synthetase into Saccharomyces cerevisiae. Transformed yeast cells were produced that exhibit efficient site‐specific incorporation of non‐biotic (...) amino acids into proteins. This makes it likely that code complexity can be elevated experimentally in mammals. BioEssays 26:111–115, 2004. © 2004 Wiley Periodicals, Inc. (shrink)