Xeroderma Pigmentosum is a rare genetic illness which principally consists in a deficiency of the Nucleotide Excision Repair (NER), the ability of human cells to repair damages provoked by exposure to ultraviolet rays, principally coming from the sun. This means patients suffering from XP have a probability of undergoing skin tumors one thousand times bigger than a healthy person. There is no cure to this disease. Ultraviolet rays, principally coming from the sun, but also produced by some artificial lighting systems, are always and in any case harmful to those who suffer from the illness. There is no secure level of exposure to ultraviolet rays and the damages are cumulative: bigger the exposure, bigger the damage. The life expectancy of a person who suffers from XP is very low. This forces patients who suffer from XP to lead a literally upside down life: where the day of a healthy child ends, the one of an XP sufferer begins. THE DARK SIDE OF THE SUN shows us precisely this reversed universe.