A handful of children around the world cannot tolerate the sun. Any exposure leads rapidly to skin cancer. They must either play indoors during daylight or be protected from head to toe in UV-proof suits. These children suffer from a strange and rare genetically-inherited disease, xeroderma pigmentosum, or XP, which means that within seconds of the sun's rays touching their skin, they are in danger of developing skin cancer. Sun Children with XP are missing the crucial gene that repairs damage to DNA and so exposure to any carcinogen - UV light, or even cigarette smoke - is lethal. Unless, they are thoroughly protected they will die from cancer at an early age. There is no cure. But these tragic children may may lead the way to new and better cancer treatment. Through studying XP sufferers, scientists have reached a whole new understanding of the genetic basis of cancer. They can now predict why one in three people will succumb to cancer. Scientists have discovered how the body survives damage and repairs itself and as a result of this, developed a radical new approach to treating cancer. Horizon explores the story of one family, where 5 out of 7 siblings suffer from XP, and how they provide the final proof that genes and DNA repair are linked to cancer. It follows an intricate 40-year scientific detective story from the discovery of DNA, through the chance findings of the cells of the XP families that led to the unexpected insight that DNA is capable of repairing itself and that the failure of this repair system underlies most cancers. After years of research, this insight is finally beginning to revolutionise medicine. Now a new concept in cancer drug therapy is just beginning medical trials based on the knowledge gained from children suffering from XP.