Marshall Nirenberg, at the National Institutes of Health, demonstrates in 1961, together with Heinrich Matthaei, the first experimental step toward deciphering the genetic code: a cell-free artificial system that translates a synthetic RNA sequence composed exclusively of uracil into a protein composed exclusively of phenylalanine, establishing the first known correspondence between an RNA base triplet — a codon — and a specific amino acid. Har Gobind Khorana, a scientist born in British India and working at the University of Wisconsin, develops methods for the chemical synthesis of RNA with precise, predetermined sequences, allowing systematic determination of which codon corresponds to each of the twenty amino acids that make up proteins. Robert Holley, at Cornell, determines in 1965 the complete structure of alanine transfer RNA, the intermediary molecule that reads the codon in messenger RNA and carries the corresponding amino acid during protein synthesis. Together, these three works complete within a few years the full dictionary of 64 codons and their corresponding amino acids, also demonstrating that the genetic code is nearly universal — shared, with minimal exceptions, by bacteria, plants, animals, and humans — powerful evidence of the common evolutionary origin of all life on Earth.