A frameshift mutation is a type of genetic mutation caused by the insertion or deletion of nucleotide bases in numbers that are not multiples of three. This is important because a cell reads a genes code in groups of three bases when making a protein. Each of these "triplet codons" corresponds to one of 20 different amino acids used to build a protein. If a mutation disrupts this normal reading frame, then the entire gene sequence following the mutation will be incorrectly read. This can result in the addition of the wrong amino acids to the protein and/or the creation of a codon that stops the protein from growing longer. Frameshift mutations can be caused by random errors during DNA replication or by exposure to certain chemicals or radiation. Some diseases that are caused by frameshift mutations include cystic fibrosis and Tay-Sachs disease.