The transformation of normal cells into cancer cells. Also referred to as carcinogenesis (although some authors make a distinction). This multi-stepped process is characterized by changes at the cellular, genetic, and epigenetic levels and abnormal cell division. Tumours enlarge because cancer cells lack the ability to balance cell division by cell death (apoptosis) and by forming their own vascular system (angiogenesis). The transformed cells lose their ability to interact with each other and exhibit uncontrolled growth, invade neighbouring tissues and eventually spread through the blood stream or the lymphatic system to distant organs. (From Mechanisms of Carcinogenesis: Contributions of Molecular Epidemiology, IARC Scientific Publication No. 157).