The existence of triplet code was simply an assumption till 1961, when Nirenberg and Methaei proved its existence by experiments. They were able to synthesize artificial mRNA, which contained only one nitrogenous base, ie, uracil. This synthetic poly $U$ sequence was then placed in a cell free system containing protein synthesizing enzymes (extracted from bacterium E. coli) and 20 amino acids together with necessary ATP. During the process, a small polypeptide molecule was produced, which was formed by the linking of phenylalanine. This suggested that UUU is the code for phenyl alanine. Nirenberg got Nobel Prize for his contributions.