Test Your Knowledge

Xeroderma pigmentosum (XP) is classified into eight complementation groups (or genetic subtypes), based upon different defects in the body’s ability to repair deoxyribonucleic acid (DNA, the molecule that encodes genetic information) damaged by ultraviolet (UV) radiation. These groups include:

• Type A (XPA). Patients with XPA have the lowest level of DNA repair and the most neurological complications (e.g., mental retardation). This type of XP is rare in the United States and most cases have been reported in Japan.
  
  
• Type B (XPB). A very rare type of xeroderma pigmentosum. Patients with XPB may also have a rare disorder characterized by a failure to grow, impaired development of the nervous system, photosensitivity and premature aging (Cockayne syndrome).
  
  
• Type C (XPC). This is the most common type of XP in the United States. Most patients with XPC rarely experience neurological problems.
  
  
• Type D (XPD). This is the second most common type of XP in the United States. Most patients with XPD often have neurological abnormalities (e.g., mental retardation, progressive deafness). Sometimes patients with XPD also have physical characteristics of Cockayne syndrome or trichothiodystrophy, which is characterized by brittle and poorly growing hair.
  
  
• Type E (XPE). This form of XP is also very rare. Patients with XPE often have mild skin symptoms and no neurological problems.
  
  
• Type F (XPF). Most cases of XPF have been reported in Japan. XPF patients may have mild skin and neurological symptoms.
  
  
• Type G (XPG). Patients with XPG may exhibit severe neurological abnormalities in addition to Cockayne syndrome.
  
  
• Variant Type (XP-V). Individuals with XP-V are clinically (but not genetically) identical to other XP patients with skin symptoms but without neurological abnormalities.

Genetic subtypes XPA, XPC, XPD and XP-V make up more than 90 percent of xeroderma pigmentosum cases worldwide, according to the Xeroderma Pigmentosum Society. Subtypes XPB, XPE, XPF and XPG are rare.