Extra DNA can appear where it does not belong. An insertion is a mutation in which one or more nucleotides are added into a DNA sequence. Small insertions of a few bases can disrupt a gene; large insertions may introduce entire genetic elements.

If the number of inserted bases is not a multiple of three, the reading frame shifts. Even in-frame insertions can alter protein structure by adding amino acids. Insertions arise from replication slippage, unequal crossing over, or mobile element activity.

Pathogenic insertions are found in many genetic disorders. Sequencing and specialized assays detect them. Some insertions are polymorphic and contribute to normal variation. Distinguishing benign from disease-causing insertions requires careful interpretation of size, location, and functional effect.

• Addition of nucleotides into a DNA sequence
• Can cause frameshifts if not in multiples of three
• Ranges from single bases to large segments
• Detected by sequencing and specialized methods