Excision endonuclease, also known as excinuclease or UV-Specific Endonuclease, is a nuclease

Nuclease

A nuclease is an enzyme capable of cleaving the phosphodiester bonds between the nucleotide subunits of nucleic acids. Older publications may use terms such as "polynucleotidase" or "nucleodepolymerase"....

 (enzyme) which excises a fragment of nucleotides during DNA repair

DNA repair

DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...

. The excinuclease cuts out a fragment by hydrolyzing two phosphodiester bonds, one on either side of the lesion in the DNA. This process is part of "nucleotide excision repair

Nucleotide excision repair

Nucleotide excision repair is a DNA repair mechanism. DNA constantly requires repair due to damage that can occur to bases from a vast variety of sources including chemicals, radiation and other mutagens...

", a mechanism that can fix specific damages to the DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 in the G1 phase

G1 phase

The G1 phase is a period in the cell cycle during interphase, before the S phase. For many cells, this phase is the major period of cell growth during its lifespan. During this stage new organelles are being synthesized, so the cell requires both structural proteins and enzymes, resulting in great...

 of the eukaryotic cell cycle

Cell cycle

The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...

. Such damages can include the thymine dimers created by UV rays.

A deficiency of excinuclease occurs in a rare autosomal recessive disease called xeroderma pigmentosum

Xeroderma pigmentosum

Xeroderma pigmentosum, or XP, is an autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet light is deficient. In extreme cases, all exposure to sunlight must be forbidden, no matter how small. Multiple basal cell carcinomas and other skin...

. Diagnosis of this disease is done by measuring the enzyme's level in white blood cells in a blood sample. Symptoms in children include extreme UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 sensitivity, excessive freckling

Freckle

For other uses of the word, see Freckles .Freckles are clusters of concentrated melanin which are most often visible on people with a fair complexion. A freckle is also called an "ephelis". Freckles do not have an increased number of melanin producing cells...

 , multiple skin cancer

Skin cancer

Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...

s and corneal ulcer

Corneal ulcer

A corneal ulcer, or ulcerative keratitis, is an inflammatory condition of the cornea involving loss of its outer layer. It is very common in dogs and is sometimes seen in cats...

ations. Typically, these symptoms are seen during a child's first sun exposure.