Skin cancer Introduction Skin cancer is usually a result of too much sun exposure. Skin cancer is the most common form of cancer. Many types of skin cancer are both preventable and treatable. There are 5 different types of skin cancer: Basal cell carcinoma is the most common form, accounting for 90% of all skin cancers. It starts in the basal cells, at the bottom of the outer skin layer. This skin cancer is caused by long-term exposure to sunlight. It is the most easily treat