A large number of genes are affected by mutations that occur when cells replicate during the process of cell division. The most common type of mutation is point mutation, which involves the substitution, insertion, or deletion of a single nucleotide base. Point mutations are responsible for most of the genetic disorders seen in humans, including sickle cell anemia and cystic fibrosis. Another type of mutation called chromosomal rearrangement can affect multiple genes at once and result in larger physical changes. Chromosomal rearrangement mutations are typically caused by errors that occur during meiosis such as crossing over or nondisjunction.