Khorana's studies also identified three chain termination signals. The existence of one of these signals became evident when a polyribonucleotide with a repeating GAUA sequence (GAUAGAUAGAUA……..) was studied. Instead of directing the synthesis of a polypeptide, this polyribonucleotide directs the synthesis of a tripeptide with the sequence lle-Asp-Arg. the UAG codon acts as a chain termination signal. Two other codons, UAA and UGA, also were shown to be termination codons. The finding of three stop codons fit nicely with the available genetic evidence. Working independently, the laboratories of Seymour Benzer and Syd-ney Brenner had demonstrated that some gene mutations cause the gene product to be much shorter than normal. They explained the production of the truncated polypeptides by proposing that the mutations change normal codons into stop codons. Mutations that change a normal codon into a stop codon are termed nonsense mutations because they change a triplet from one that specifies an aminoacid to one that does not. Three different types of nonsense mutation were shown to exist and geneticists named them amber, ochre, and opal. These names, which have no physiological significance, were coined in a rather lighthearted spirit. The amber codon is UAG; the ochre codon is UAA; and the opal codon is UGA.