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Marshall Nirenberg and J.H. Matthaei observe that a synthetic polynucleotide, poly U, directs the synthesis of a polypeptide composed only of phenylalanine. They conclude that the nucleotide base triplet UUU must code for phenylalanine. This is the start of successful efforts to decipher the genetic code. With Robert Holley and Har Gobind Khorana, Nirenberg is awarded the Nobel Prize in Medicine or Physiology in 1968.