The 1961 Nirenberg–Matthaei experiment showed that synthetic poly-U RNA directed the production of phenylalanine.
Marshall Nirenberg and J. Heinrich Matthaei used a cell-free extract from Escherichia coli, allowing them to study protein synthesis without intact living cells. They added an artificial RNA molecule made entirely of uracil bases, known as poly-U RNA, along with the components required for translation.
The extract produced a polypeptide consisting entirely of phenylalanine. This demonstrated that a sequence of three uracil bases, UUU, functions as a codon for phenylalanine. It was the first codon assignment in the genetic code and a major step toward decoding how nucleotide sequences specify proteins.
A common mix-up is confusing poly-U with the entire genetic code. The experiment identified one codon relationship, not every codon at once. Later work by Nirenberg, Har Gobind Khorana, Robert Holley, and others established the broader code. The genetic code is largely shared across organisms, although mitochondria and some microbes have documented variations.