Herbie is a typical American college freshman who loves to eat. His second most favorite food is hotdogs. He cuts them into his Kraft macaroni and cheese at a 1:1 ratio, going through a score of hotdogs each week. He doesn't know his brand is preserved with sodium nitrite, but his stomach is slowly finding out as it's acid converts the sodium nitrite to nitrous acid, a weak mutagen. In the lining of Herbie's stomach three little cells each experience a mutation in their DNA and it happens to be in the very same gene--a kinase gene called src. (Src is an example of what Dr. Howard Temin called a "proto-oncogene.) But the three little cells each experienced a different kind of mutation in that gene:
Cell #1's src DNA was altered so that it's product is present in normal amounts, but no longer functions.
Cell #2's src DNA was altered so that it produces almost no gene product at all.
Cell #3's src DNA was altered so that it's gene product has normal function but is overproduced.
Workers have long known that mutations cause cancer. A mutagen (substance or form of energy that causes mutations) is usually a carcinogen (substance or form of energy that causes cancer).