Skin cancer. Light micrograph of a section through skin with basal cell carcinoma, a common skin tumour that arises from the basal cells of the epidermis. Although the cancer behaves malignantly by invading the underlying dermis it almost never metastasises (spreads). Cells within the tumour (purple) are haphazardly arranged. The skin surface is at top (red). Basal cell carcinomas occur as lesions on areas of skin exposed to light, particularly the face. The lesions are commonly known as rodent ulcers.