Basal cell carcinomas (BCC) occur when a mutation develops in one of your skin's basal cells. Basal cells are found at the bottom of the epidermis –the outermost layer of your skin. Basal cells produce new skin cells as old ones near the surface die off. Sun exposure is the main cause of damage leading to BCC. As a result, almost all BCCs occur on parts of your body that get the most sun.
Image source DermNet, NZ. Also see more images of basal cell carcinoma at DermNet NZ(external link)