An individual with the disorder Xeroderma pigmentosum is hypersensitive to sunlight succumbs to DNA damage caused by light exposure. What is the probable cause for this disorder?

Xeroderma pigmentosum is a genetic disorder and the person suffering from this disorder is sensitive to ultraviolet light due to the disability of the enzymes involved in nucleotide excision repair which prevents DNA damage caused by ultraviolet light.
Thus, the individuals are sensitive to the exposure of UV and suffer severe problems when exposed to sunlight.
The major cause of the disorder is the inability to repair DNA damage caused by exposure to sunlight due to the mutation in an enzyme involved in nucleotide excision repair.
Nucleotide excision repair is a DNA repair process that can excise out single-stranded DNA that is damaged by UV.
UV exposure leads to the addition of bulky adducts in the DNA known as thymine dimers. The enzymes of NER are involved in the removal of these adducts by excising out a segment of DNA that contains such lesions.
However, in the case of XP, the mutations make this repair system non-functional or partially functional and thus, the individual becomes highly sensitive to UV exposure.