There are numerous types of cancer, each classified according to the type of cell originally affected. Cancer harms the body when damaged cells divide uncontrollably to form clumps of tissue. These masses are known as tumors. The case of leukemia is an exception. Leukemia is characterized by a cancer that prevents normal blood function due to irregular cell division in the bloodstream. Tumors can grow and interfere with the nervous, circulatory, and digestive systems. They can change the function of the body by releasing hormones. Tumors considered benign tend to show very little growth and remain in one place. Malignant tumors form when two things happen. First, in a process called invasion, a cancer cell destroys healthy tissue by moving throughout the body using the lymphatic or blood system. In a process called angiogenesis the cell is able to divide and grow, creating new blood vessels for nourishment. A tumor is said to have metastasized when it successfully spreads to other parts of the body and grows, destroying other healthy tissue. The resulting metastasis is a serious condition that is very difficult to treat. Regulation of gene expression refers to the control of the amount and timing of appearance of the functional product of a gene. A cell must produce the genetic products it needs when it needs them. This is why control of expression is so important. This allows cells the flexibility to adapt to a changing environment, damage to the cell, external signals, and so on. More generally, genetic regulation gives the cell control over all structure and function, and is the basis for cellular differentiation, morphogenesis, and the versatility and adaptability of any organism. In general, gene expression is regulated through changes in the number and type of interactions between molecules that collectively influence DNA transcription and DNA translation..