XP repair proteins
The XPA protein acts during NER as a scaffold for assembly of other DNA repair proteins at sites of DNA damage to ensure appropriate excision of the damage.The XPB (ERCC3) protein is employed in unwinding the DNA double helix after DNA damage is initially recognized. Mutations in the XPB(ERCC3) gene can lead to XP or XP combined with Cockayne syndrome.The XPC protein forms a complex with RAD23B protein to form the initial damage recognition factor in global genomic nucleotide excision repair (GG-NER). This complex recognizes a wide variety of damages that thermodynamically destabilize DNA duplexes.The XPD (ERCC2) protein, in combination with the XPB helicase-containing transcription/repair complex TFIIH, is employed in unwinding the DNA duplex after damage is initially recognized. Mutations in the XPD(ERCC2) gene cause a variety of syndromes; XP, trichothiodystrophy (TTD), or a combination of XP and Cockayne syndrome (XPCS). Both trichothiodystrophy and Cockayne syndrome display features of premature aging, suggesting an association between deficient DNA repair and premature aging (see DNA damage theory of aging). XPE is a heterodimeric protein composed of two subunits. The larger subunit DDB1 primarily functions as a core component of CUL4A- and CUL4B-based E3 ubiquitin ligase complexes. Substrates that are ubiquitinnated by these complexes include proteins employed in DNA repair.The XPF (ERCC4) protein together with the ERCC1 protein forms a complex usually designated ERCC1-XPF. This complex separates the DNA helix for a short distance on either side of the site of damage.
It then acts as an endonuclease to incise the damaged DNA strand on the 5 side of the damaged site. Mutant cells with deficient ERCC1-XPF are not only defective in NER, but also in the repair of double-strand breaks and inter-strand crosslinks. The XPG protein is an endonuclease that incises DNA during NER at the 3 side of the damaged nucleotide. Mutations in the XPG (ERCC5) gene can lead to XP alone, or in combination with Cockayne syndrome (CS), or in combination with infantile lethal cerebro-oculo-facio-skeletal syndrome. Diagnosis
Types
There are seven complementation groups, plus one variant form:

Treatment
There is no cure for the disorder; all treatment is symptomatic or preventive. Symptoms can be avoided or controlled by completely avoiding exposure to sunlight, either by staying indoors or wearing protective clothing and using sunscreen when outdoors. Keratosis can also be treated by using cryotherapy or fluorouracil. In more severe cases of XP, even minuscule amounts of UV light, for example, from covered windows or fluorescent bulbs, can be very dangerous and trigger symptoms.On September 10, 2020, Clinuvel Pharmaceuticals announced that it was investigating the use of its FDA-approved flagship drug Scenesse as a potential treatment to increase pain-free light exposure for patients with xeroderma pigmentosum.