An oncogene that is believed to be responsible for up to 90% of all human pancreatic cancer, 50% of human colon cancers, 40% of lung cancers, and 30% of leukemias. The ras gene is present in the DNA of all human tissues, and codes for ras proteins, which help to signal each cell to divide and grow at appropriate time(s). When the ras gene has been damaged or mutated (e.g., via exposure to cigarette smoke or ultraviolet light), it codes for (i.e., causes to be manufactured in the cell’s ribosome) a mutated version of the ras gene that can cause the cell to become cancerous (i.e., divide and grow uncontrollably).