Cancer is a genetic disease caused by changes on genes that control critical cell functions. Normal cell proliferation is tightly regulated with a balance between the activities of proto-oncogenes and tumor suppressors.For example, DNA damage and repair mechanisms exist in order to decrease the likelihood of genetic mutation and cell transformation. Apoptosis is needed to destroy cells that represent a threat to the integrity of the organism. In addition, the immune system is ready to recognize and destroy cancerous cells. However these error-correction methods often fail in small ways, especially in environments that make errors more likely to arise and propagate. Accumulating disruptions in these control mechanisms lead to progressive error accumulation until unregulated proliferation and cancer forming. The disruption of this homeostasis leads to deregulated cell proliferation in cancer. Unlike normal cells which mature into very distinct cell types, cancer cells are less differentiated and invasive and often gain the function to avoid programmed cell death, or apoptosis, which the body uses to get rid of unneeded cells. In addition, Cancer cells are also able to evade the immune system, which normally removes damaged or abnormal cells from the body. Cancer cells will influence the microenvironment to promote angiogenesis which supply tumors with oxygen and nutrients. Metastatic tumors cause severe damage to normal body functions and result from the spread of cancer cells from the primary site to another part of the body. Cancer types are usually named for the organs or tissues where the cancers form though they can also be described by the type of cells that formed them.