The simplest form of mutation is a single DNA base change, frequently referred to as a “single nucleotide polymorphism” (SNP). Strictly, this term should only be applied to single base changes that are observed in at least 1% of a “normal” population. However, it is frequently used to refer to any single base mutation and is used in that context here. Many SNPs occur in noncoding regions of DNA, where they may affect transcription, mRNA splicing, or mRNA stability. When a single base change occurs in an exon, it will fall into one of three classes: (1) a “synonymous” mutation which does not change the amino acid sequence of the resultant protein (although this may still affect expression, splicing, or mRNA stability), (2) a “nonsense” mutation resulting in premature termination of the protein sequence, or (3) a “non-synonymous” (or “missense”) mutation (an nsSNP) resulting in a single amino acid change. At the protein level, an nsSNP results in a “single amino acid..