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  1. There are a variety of different types of mutations in the human genome and many diverse mechanisms for their generation.

  2. Single-base-pair substitutions account for the majority of gene defects. Among them, the hypermutability of CpG dinucleotides represents the most important and frequent cause of mutation in humans.

  3. Point mutations may affect transcription and translation, as well as mRNA splicing and processing. Mutations in regulatory elements are of particular significance, since they often reveal the existence of DNA domains that are bound by regulatory proteins. Similarly, mutations that affect mRNA splicing can contribute to our understanding of the splicing mechanism.

  4. We describe mechanisms of gene deletion and the DNA sequences that may predispose to such lesions, as well as potential mechanisms underlying insertions, duplications, or inversions, with representative examples.

  5. Retrotransposition is a rare but biologically fascinating phenomenon that can lead to abnormal phenotypes if the double-stranded DNA is inserted in functionally important regions of a gene. Long interspersed repeat elements (LINEs) and Alu repetitive elements and pseudogenes have been shown to function as retrotransposons, and their de novo insertion in the genome can produce disease.

  6. The expansion of trinucleotide repeats represents a relatively novel category of mutations in humans. There is a growing list of disorders that result from an abnormal copy number of trinucleotides within the 5′ or 3′ untranslated regions, coding sequences, and introns of genes. The pathophysiologic effects of the expansion of the trinucleotide repeat are unknown. Additionally, at least one disorder is caused by expansion of a 12mer repeat (progressive myoclonus epilepsy).

  7. The study of mutations in human genes is of paramount importance in understanding the pathophysiology of hereditary disorders, in providing improved diagnostic tests, and in designing appropriate therapeutic approaches.

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