The three-nucleotide sequence elements on mRNA that code for individual amino acids are called codons. These are matched by anti-codons on tRNA to ensure that the appropriate amino acid as aligned with a given mRNA sequence. The 64 possible combinations of A, C, G, and T at each codon code for only 20 different amino acids. This redundancy in the genetic code, permitting multiple codons to specify common amino acids, is considered a form of protection against DNA mutations and has applications in identifying foreign DNA from sources such as viruses which use different 'dialects' of the genetic code. The chemical characteristics of the amino acids in a protein chain cause it to fold into a defined structure. The structure of a protein determines its function. Because the DNA sequence of a gene, through an mRNA intermediate, dictates the sequence of amino acids in a protein, and the sequence of amino acids in a protein determines its structure, one can deduce a protein sequence, and potentially (but not yet) its structure and function, from the gene sequence encoding it.