Normal cell growth and division involve a number of cell cycle phases. Each cell cycle is regulated by a number of factors such as growth factors, spatial availability, or regulation by the cell itself. When carcinogenesis occurs, the once-normal cell becomes cancerous. A carcinogen (I.e., environmental pollutant) attacks a normal cell, causing it to become mutated. The mutated cell is capable of uncontrolled cell division and tumor development. The tumor is named according to the tissue or organ where it originated. Basic classes of cancer types include carcinomas, melanomas, sarcomas, leukemia, and lymphomas. The most common tumors are carcinomas; sarcomas are the rarest. The treatment for the various types of cancers is dependent on the evaluation of the tumor. Tumors are graded and staged to determine their severity and progression. Grading involves determination of the microscopic appearance of the tumor; staging involves clinical and pathological evaluation. The most common and widely used staging method is called the TNM systems, whereby the tumor size (T), the extent of the involvement of regional lymph nodes (N), and the presence or absencs of distant spreading or metastasis (M) are determined. Various types of cancers are staged in different ways.