Silent nucleotide substitutions in evolution are found by comparing homologous sequences of DNA from different organisms. Silent changes are common in the third bases of codons, so that no changes takes place in the specified amino acid. Silent changes average about half of the total nucleotide substitutions during evolution of protein-coding regions of genes. Nucleotide substitutions also take place during evolution in non-coding regions of DNA, such as in intervening sequences and in sequences that precede and follow codon regions in genes. Deletions and additions from events of recombination, in addition to nucleotide substitutions, are common in these non-coding regions.