RECENT advances in methods for tRNA purification and RNA sequence determination have aided in the elucidation of the sequences of approximately thirty tRNAs. Studies with tRNAs chemically altered in vitro, or by mutation, are providing insights into aspects of the recognition of mRNA codons by tRNAs1–3, the specificity of protein-nucleic acid interactions1,4,5 and the mechanisms of post-transcriptional modification of RNAs6,7.