Many genes determine when and how a cell grows. In general, there are two groups of genes that affect cell growth. You can think of these genes as traffic lights that control cell growth. = Tumor Suppressor Genes These genes normally "STOP" or prevent cell growth (think of these genes as a red light). Loss of tumor suppressor genes promotes cancer cell growth. = Proto-oncogenes These genes normally promote cell growth (think of these genes as a green light). Increased expression or activity of proto-oncogenes results in oncogenes, which instruct cancer cells to divide.