When the growth factor message reaches the cell nucleus, it activates genes called proto-oncogenes. These genes produce proteins that stimulate the cell to divide. In cancerous cells, mutations in proto-oncogenes cause these genes to malfunction. When a proto-oncogene mutates, it becomes an oncogene—a gene that instructs the cell to grow and divide repeatedly without stimulation from neighboring cells.
Some oncogenes overproduce growth factors, causing the cell to divide too often. Other oncogenes stimulate the cell to reproduce even when no growth factor is present. Cancer researchers have identified about 100 different types of proto-oncogenes and their cancer-causing oncogene counterparts. The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.