In the World Health Organisation’s Classification of Skin tumors in 2006, literally hundreds of benign and malignant tumors are catalogued. Fortunately, most of the malignant tumors, (apart from BCC, SCC, and Melanoma) are rare.
The critical events which lead to the development of naevi remain a mystery. Exposure to ultraviolet radiation and genetic factors both play a role. There appears to be a variable relationship between skin, eye and hair colour, the tendency to sunburn and freckle, and the development of naevi.