How do insertions and deletions cause frameshift mutations?

In genetics, a frameshift mutation occurs when the number of nucleotides inserted into or deleted from a DNA sequence is not divisible by three.1

In a protein-coding gene, the genetic code is read in triplets called codons, which define the "reading frame" for protein synthesis. When an insertion or deletion involves a number of nucleotides that is not a multiple of three, it shifts this reading frame.231

Because the cellular machinery continues to read the sequence in groups of three, this shift alters every codon downstream of the mutation site. Consequently, while the portion of the protein before the mutation remains unchanged, the remainder of the sequence is translated into a completely different—and often functionally meaningless—string of amino acids. This typically results in a drastic loss of protein function.23