A child with xeroderma pigmentosum develops skin cancers before age 10. XP is nucleotide excision repair failure — UV-damaged DNA (thymine dimers) can't be cut out and fixed, so mutations accumulate with every sun exposure: freckling, then melanomas and carcinomas in childhood. Management is extreme sun avoidance — the disease proves repair's importance: we all sustain thousands of DNA lesions daily, and repair enzymes are the silent guardians. p53's story is parallel — the 'guardian of the genome' mutated in most human cancers.