text: Nonsense mutations include stop-gain and start-loss. Stop-gain is a mutation that results in a premature termination codon (a stop was gained), which signals the end of translation.  This interruption causes the protein to be abnormally shortened. The number of amino acids lost mediates the impact on the proteins functionality and whether it will function whatsoever. Stop-loss is a mutation in the original termination codon (a stop was lost), resulting in abnormal extension of a proteins carboxyl terminus. Start-gain creates an AUG start codon upstream of the original start site. If the new AUG is near the original start site, in-frame within the processed transcript and downstream to a ribosomal binding site, it can be used to initiate translation. The likely effect is additional amino acids added to the amino terminus of the original protein. Frame-shift mutations are also possible in start-gain mutations, but typically do not affect translation of the original protein. Start-loss is a point mutation in a transcript's AUG start codon, resulting in the reduction or elimination of protein production.
  Missense mutations code for a different amino acid.  A missense mutation changes a codon so that a different protein is created, a non-synonymous change. Conservative mutations result in an amino acid change.  However, the properties of the amino acid remain the same (e.g., hydrophobic, hydrophilic, etc.).  At times, a change to one amino acid in the protein is not detrimental to the organism as a whole.  Most proteins can withstand one or two point mutations before their function changes. Non-conservative mutations result in an amino acid change that has different properties than the wild type.  The protein may lose its function, which can result in a disease in the organism. For example, sickle-cell disease is caused by a single point mutation (a missense mutation) in the beta-hemoglobin gene that converts a GAG codon into GUG, which encodes the amino acid valine rather than glutamic acid. The protein may also exhibit a "gain of function" or become activated, such is the case with the mutation changing a valine to glutamic acid in the BRAF gene; this leads to an activation of the RAF protein which causes unlimited proliferative signalling in cancer cells. These are both examples of a non-conservative (missense) mutation.
  Silent mutations code for the same amino acid (a "synonymous substitution").  A silent mutation does not affect the functioning of the protein. A single nucleotide can change, but the new codon specifies the same amino acid, resulting in an unmutated protein.  This type of change is called synonymous change since the old and new codon code for the same amino acid.  This is possible because 64 codons specify only 20 amino acids.  Different codons can lead to differential protein expression levels, however.