Skin cancer. Coloured transmission electron micrograph (TEM) of a section through cancerous skin tissue showing filaments of keratin (red). Keratin is the protein found in skin and other tissues like hair and nails. This tissue is from a squamous cell carcinoma, a highly malignant tumour developing from squamous cells found in the epithelial layers of the body. The tumour forms interconnecting bands of keratinised cells (red) that invade the surrounding tissue. Invasion of the adjacent tissue is one of the features of a malignant tumour, as opposed to a benign tumour.