George Gamow in 1954, pointed out the possibility of a three-letter code i.e. triplet codon.
This will give 4 x 4 x 4 = 64 codons which are more than enough to code for twenty amino acids.
Out of these 64 codons, 3 codons are stop codons.
Hence, 61 effective codons are there for the synthesis of twenty amino acids.