DNA formed of two single polynucleotide strands wrapped around each other forming a double helix.Growth factors constitute an ill defined set of polypeptides that can modulate cell function and exert specific and potent growth regulatory action on target cells. The development of cancer is a complex process that requires the accumulation of damage to the cell's growth-controlling genes, including damage to tumor suppressor genes and proto-oncogenes.Tumor suppressor genes are normal genes that slow down cell division, repair DNA mistakes, and tell cells when to die. When tumor suppressor genes don’t work properly, cells can grow out of control, which can lead to cancer. Tumor suppressor genes code for proteins that serve as the "stop" signals that tell a cell to leave the cell cycle and stop dividing. Proto-oncogenes code for the go signals that tell the cell to stay in the cell cycle and continue to divide. Many tumor suppressor genes have been identified.Several mutations in oncogenes and tumor suppressor genes have been found often enough to be useful in helping decide which people are at higher risk for developing certain types of cancers.