The first codon discovered by Nirenberg and Matthaei was :

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The Nirenberg and Matthaei experiment was performed by American biologist Marshall W. Nirenberg (1927-2010) and German biologist J. Heinrich Matthaei in 1961. Matthaeiat was a postdoc fellow in the lab of Nirenberg at the National Institutes of Health, USA. The Nirenberg and Matthaei experiment was a famous experiment used to decipher the genetic code.

In this experiment,  Nirenberg and Matthaei used bacterial cell extract that can synthesize protein in an in vitro condition. They used artificial RNA consisting of only Uracil nucleotides (polyuridine bases, polyuridylic acid, or poly-U). When the mRNA containing poly-U nucleotides to the cell-free extract, a protein was formed consisting of only phenylalanine amino acid. The triplet codon UUU codes for phenylalanine.