A type of point mutation in which a single nucleotide is substituted with (or exchanged for) a different nucleotide that may result in an altered sequence of amino acid during translation, which may render the newly synthesized protein ineffective.
Examples of base-pair substitutions: purine substituted with a different purine (A → G) or a pyrimidine for a different pyrimidine (C → T). This type of substitution mutation is transition. Another kind is transversion, which is less common. In transversion, purine is substituted with pyrimidine or a pyrimidine with a purine.
See also: point mutation.