A genetic disease or disorder is the result of changes, or mutations, in an individual’s DNA. A mutation is a change in the letters (DNA sequence) that make up a gene. This is sometimes referred to as a “spelling” mistake. Genes code for proteins, the molecules that carry out most of the work, perform most life functions, and make up the majority of cellular structures. When a gene is mutated so that its protein product can no longer carry out its normal function, a disorder can result.