Many enzymes are regulated by the attachment of phosphate groups to sidechains of specific Serine , Threonine , or Tyrosine amino acids by Protein Kinases , enzymes which transfer phosphate groups from ATP to proteins. The Protein Kinases which regulate the Cell Cycle are themselves regulated by interaction with protein molecules called Cyclins . Division of cells in multicellular organisms is controlled by Growth Factors which are secreted by other cells. Different types of cells require different growth factors or combinations of growth factors which take the cell past the Restriction Point and lead to cell division. Loss of this control of cell division in multicellular organisms can lead to inappropriate cell division forming a tumor .
Cancer is a condition in which cells from a tumor can escape and start new tumors in other tissues.
Not all tumors are cancer however.