Combined with the technology to generate trinucleotides of known sequence, Nirenberg’s assay provided a way to assign each specific amino acid to one or more specific trinucleotides. Within a few years, the genetic code was cracked, all 20 amino acids were assigned at least one trinucleotide, and 61 of the 64 trinucleotides were found to correspond to an amino acid. The final three trinucleotides, now known as “stop” codons, signal termination of protein synthesis.