Cancer is a term used for malignant tumors that share the common feature of uncontrolled cell proliferation. Cancers are usually classified on the basis of the tissue of origin. By this criterion, cancers can be divided into five main categories: carcinoma, sarcoma, myeloma, leukemia and lymphoma. Carcinomas refer to malignant growths of epithelial origin or cancers of the internal or external lining of the body. These constitute almost 90% of all malignant tumors and can be further classified as adenocarcinomas: which develop in an organ or a gland; squamous cell carcinomas: which originate in the squamous epithelium; and, basal cell carcinomas: which arise in the basal cells of the epithelium. Sarcomas are cancers that originate in the supportive or connective tissues of the body. Carcinomas and sarcomas are generally referred to as solid tumors. Liquid tumors or blood cancers include myelomas: cancers developing in the plasma cells of the bone marrow; leukemias: cancers of blood or bone marrow, which are often associated with abnormally large numbers of immature white blood cells; and lymphomas: cancers that develop in the nodes or glands of the lymphatic system.