What happens when dna is changed to mutant dna?

A mutation in the DNA sequence occurs during cellular mitosis when a cell replicates. These mutation of cause cancer cells. Granted, some mutations can be helpful. For example, there is a mutation in about 1% of the population that actually causes the person with the mutation to be "immune" to HIV. This mutation causes the receptor proteins of T cells to be able to block the HIV virus and thus prevents it from entering the cell. Cool eh?