As a first step in deciphering the genetic code, short artificial mRNA
   homopolymer  sequences  of UUUUUU ..., AAAAAA ..., or CCCCCC were used
   as the template to synthesize polypeptides using radioactively labeled
   amino acids.

   Poly  U  was  found to incorporate ^14C-phenylalanine, indicating that
   the codon for phenylalanine is UUU.