substitution mutationa mutation affecting the base sequence of a DNA molecule, in which one base is substituted for another one, with no loss or gain of base and therefore no risk of a FRAMESHIFT.
A transition substitution mutation is the replacement of a PYRIMIDINE base by another pyrimidine (e.g. thymine changing to cytosine) or a PURINE by another purine (e.g. adenine replacing guanine). A transversion substitution mutation is the replacement of a pyrimidine by a purine or vice versa.