Missense mutations 

These are mutations in which a mutation in the genetic code results in a change in the protein building block (amino acid) that is produced. 

What a missense mutation means in terms of disease depends very much on where the mutation occurs. 

Example 1: If a mutation occurs in an amino acid that is vital for how the protein folds, a missense mutation may completely disrupt the shape of the protein, preventing it from interacting with other proteins required to carry out its normal function. This would lead to a loss of activity.

Example 2: If a mutation occurs in an amino acid that only slightly changes how a protein folds, therefore retaining some though not all of its ability to interact with other proteins. This would lead to a reduction in activity, but some normal function can still be carried out.