Skin cancer cell. Coloured scanning electron micrograph (SEM) of a section through a squamous cell carcinoma cell. This cell has recently undergone mitosis (nuclear division), as it has two nuclei (purple). The next stage is cytokinesis (cellular division) to produce two daughter cells. The large nuclei are typical of cancer cells. Seen within each nucleus is a nucleolus (darker purple), the site of ribosome production. Squamous cell carcinoma is the second most common form of skin cancer and affects cells in the upper layers of the skin. Most cases arise from pre-cancerous lesions known as solar keratoses, which are caused by sun damage.