An alteration in the sequence of DNA bases read as base triplets during transcription, caused by the deletion (or addition) of a single nucleotide in the DNA sequence. The missing (or additional) base results in an abnormal triplet and also causes each subsequent triplet group to be altered. A frameshift mutation therefore produces messenger RNA with corresponding changed codons and results in the synthesis of an abnormal, and usually nonfunctional, protein. For example, the deletion of uracil (U) in the second codon of the following mRNA sequence:

AUU CAU CGG UAG ACC UGU AUG

results in the frameshifted sequence:

AUU CAC GGU AGA CCU GUA UG