The second cardinal feature of Bloom's syndrome is the development of a rash upon exposure to sunlight. The component of sunlight that causes the rash is ultraviolet light, because light in the visible range does not produce the rash. Ultraviolet light, specifically radiation in the 290-320 nanometers range damages DNA to produce specific lesions. The DNA damage activates a DNA repair system that is known to be working fine in Bloom's syndrome. Is there a type of UV damage for which the Bloom's syndrome gene is needed? What is the cause of the rash? The rash contains cells from the immune system. What causes these immune cells to appear at the site of the lesion or to become active there? Could the stem cell* populations in the skin have trouble in keeping up and replacing cells that are damaged by the sun?
We know it is important to protect the skin from sun exposure with sunscreen, clothing, a hat; generalizable knowledge about the mechanisms underlying the reaction to sunlight could help us deal more effectively with the reaction when it occurs. If the problem traced to stem cells, maybe we could learn how to preserve them better so that tissue renewal after UV damage could be more robust.
In Bloom's syndrome, there are other types of skin lesions, such as café au lait spots and adjacent hypo- and hyper-pigmented spots, that can be present anywhere on the body and that do not trace to sun exposure. These lesions could have different sorts of origins, some tracing to aberrations during fetal development and others to the maintenance phase of life; the lesions do not appear to be anything to complain about, although we recognize that cancers of the skin are very common in light-skinned populations, and they are common in Bloom's syndrome too.
*For more information about stem cells, please see Small Body Size.