Single nucleotide polymorphisms exist at a high frequency in the human genome. Estimates of variability suggest that up to 1 in 1000 bp within coding regions of the genome are polymorphic, and the frequency of polymorphism within noncoding regions is even higher at about 1 in 500. Because of redundancy in amino acid coding, not all single-base substitutions result in an altered amino acid sequence within a gene. However, even degenerate polymorphisms can potentially alter transcriptional or translational activity. Polymorphisms within promoter or enhancer regions can interfere with transcription factor binding sites and, therefore, can also alter transcriptional activity.