Skin cancer cells proliferate in this model that shows stages of the disease. Specific mutations (red cells) slowly expand and multiply to give rise to benign tumors in the epidermis or in melanocytes. Eventually, they can develop to become squamous carcinoma or melanoma and metastasize to other organs (lymph nodes, brain or lung). To understand the mechanisms underlying promotion and progression of melanoma and non-melanoma skin cancer, researchers mimic the entire process in an animal model (for example, in mice). They use a tumor initiator followed by applications of agents that induce tumor formation, promotion and progression to invasive carcinoma. Inflammatory cells that promote tumor development are shown as blue dots. (Illustration: Indra laboratory)