Mutations can occur anywhere in DNA. If a mutation occurs in the coding region of a gene, it can change the amino acid sequence of the protein (the gene product), compromising the protein's function. A change in a protein's amino acid sequence can occur in a critical region, such as a catalytic domain, often leading to reduced protein function. The linear protein below shows one possible location of a change in an amino acid sequence. Several types of mutations are possible.

A silent mutation does not usually affect protein function, because the nucleotide substitution results in a codon that calls for the same amino acid as found in the normal protein.

Missense mutations cause one amino acid to substitute for another in a protein. The resulting protein may be defective, but if the substituted amino acid is similar enough to the original, the change in protein function may be minor.

A nonsense mutation involves a base substitution that causes a stop codon to occur prematurely in the mRNA, resulting in a shortened, and typically nonfunctional, protein. However, if the nonsense mutation is found very near the end of the coding sequence, the protein may still be functional.

In frame-shift mutations, nucleotides may be inserted into, or deleted from, a gene. The reading frame of
the mRNA is altered during translation, almost always leading
to nonfunctional proteins.

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