frameshift mutation

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A mutation in a DNA chain that occurs when the number of nucleotides inserted or deleted is not a multiple of three, so that every codon beyond the point of insertion or deletion is read incorrectly during translation.

Wiktionary

1. n. genetics a genetic mutation that either inserts or deletes a series of nucleotides that is not divisible by three and thus disrupts gene expression by codons