Squamous cell carcinoma of the skin occurs when the flat, thin squamous cells in the middle and outer layers of your skin develop changes (mutations) in their DNA. A cell's DNA contains the instructions that tell a cell what to do. The mutations tell the squamous cells to grow out of control and to continue living when normal cells would die. Most of the DNA mutations in skin cells are caused by ultraviolet (UV) radiation found in sunlight and in commercial tanning lamps and tanning beds. But sun exposure doesn't explain skin cancers that develop on skin not ordinarily exposed to sunlight. This indicates that other factors may contribute to your risk of skin cancer, such as having a condition that weakens your immune system.