A missense mutation occurs when there is a point mutation in a gene, which substitutes a different codon, and subsequently a different amino acid. A base pair substitution occurs when one DNA base pair replaces another. A nonsense mutations is a point mutation that causes a premature chain termination or stop codon, resulting in a truncated and typically non-functional protein. A frameshift mutation occurs when base pairs are either inserted into or deleted from DNA, causing a shift in the reading frame.