Final answer:
A frameshift mutation, caused by an insertion or deletion of nucleotides that is not a multiple of three, leads to a nonfunctional protein due to an altered reading frame and disrupted amino acid sequence.
Step-by-step explanation:
A frameshift mutation, specifically caused by an insertion or deletion of nucleotides that is not a multiple of three, nearly always results in a nonfunctional protein. This is because the change in reading frame alters every amino acid after the point of the mutation, leading to a significant disruption in the protein's structure and function.