In genetics, missense mutations or nonsynonymous mutations are types of point mutations where a single nucleotide is changed to cause substitution of a different amino acid. This in turn can render the resulting protein nonfunctional. Such mutations are responsible for diseases such as Epidermolysis bullosa, sickle-cell disease, and SOD1 mediated ALS(Boillée 2006, p. 39).
For example, in sickle-cell disease, the 17th nucleotide of the gene for the beta chain of hemoglobin found on chromosome 11 is erroneously changed from the codon GAG (for glutamic acid) to GTG (which codes valine), so the 6th amino acid is incorrectly substituted.
Not all missense mutations lead to appreciable protein changes. An amino acid may be replaced by an amino acid of very similar chemical properties, in which case, the protein may still function normally; this is termed a neutral or "quiet" mutation. When an amino acid may be encoded by more than one codon (so-called "degenerate coding") a mutation in a codon may not produce any change in translation; this would be a synonymous mutation and not a missense mutation.
Boillée, Séverine (2006), "ALS: A Disease of Motor Neurons and Their Nonneuronal Neighbors", Neuron, 52 (1): 39–59.