Any rearrangement within a gene is a mutation, and such rearrangement is usually due to a random genetic accident. The mutation can be harmless and simply add to the variation within an organism’s genome, or it can be harmful with dire consequences.

Mutations may occur within the gametic cells (germinal mutation) or within any nonsex cell (somatic mutation). The difference is that a germinal mutation will affect all cells of an individual, while a somatic mutation will only affect those cells produced by mitosis from the original mutated cell. The following are various categories of mutations: