An insertion or deletion involving a number of base pairs that is not a multiple of three, which consequently disrupts the triplet reading frame of a DNA sequence. Such mutations usually lead to the creation of a premature termination (stop) codon, and result in a truncated (shorter-than-normal) protein product. OR Insertions or deletions of genetic material that lead to a shift in the translation of the reading frameThe mutation usually leads to nonfunctional proteins.