Final answer:
Patients with xeroderma pigment sum have a defect in the genes involved in nucleotide excision repair, leading to increased risk of skin cancer.
Step-by-step explanation:
Patients with the classical form of xeroderma pigment sum have a defect in one of the genes involved in nucleotide excision repair. Xeroderma pigment sum is a genetic disorder that impairs the body's ability to repair DNA damage caused by ultraviolet light. When individuals with this condition are exposed to UV light, thymine dimers are formed, which distort the structure of DNA and increase the risk of contracting skin cancer.