Gene mutations describe a change in the DNA sequence that makes up a gene. There are many kinds of gene mutations which have been characterized by scientists. The main categories of mutations are germline mutations, which occur in the gametes (haploid male or female reproductive cells), and somatic mutations, occur in all other cell types. Chromosomal alterations are mutations that change the structure of chromosomes in a variety of ways such as deletion, duplication, insertion, translocation, or inversion of a section of the chromosome. Point mutations include a single nucleotide change within the DNA and these mutations can be classified as silent (the mutation codes for the same amino acid so the change has no effect), missense (the mutation codes for an alternative amino acid), and nonsense (the mutation codes for a premature stop codon). Frameshift mutations occur when the addition or deletion of nucleotide(s) alter the reading frame of the sequence altering all subsequent amino acid coding.