Are there any exceptions to the universality of genetic code?

The few rare exceptions to the universal genetic code are mainly found in mitochondrial genomes. These exceptions involve changes in the typical interpretation of certain codons. For example, in mammals and fruit flies, codons like AGA and AGG (which usually signify the amino acid arginine) instead act as stop codons, signaling the end of protein synthesis. Similarly, the codon UGA, which normally serves as a stop signal, codes for the amino acid tryptophan. Additional exceptions occur in protozoans and bacteria, where stop codons can code for amino acids such as tryptophan in Mycoplasma capricolum bacteria. However, these exceptions do not indicate that life originated from multiple sources.