Squamous cell carcinoma cells. Fluorescence light micrograph of skin cancer cells in a case of squamous cell carcinoma, the second most common form of skin cancer. The cell nuclei are blue. The other colours highlight various cellular proteins. This cancer arises from squamous cells, flat scale-like cells in the upper layers of the skin. The tumour forms interconnecting bands of keratinised cells 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.