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, "quiet", "silent" or conservative mutation. alternatively, the amino acid substitution could occur in a region of the protein which does not significantly affect the protein secondary structure or function. 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 substitution and not a missense mutation.