The third part of the test involves injecting Strain A mice with the chemical, then looking for the development of cancerous or pre-cancerous tumors in the lungs after about six months. In each case, animals exposed to the chemical would be compared to control animals, which are not given the chemical. But some researchers oppose the use of Strain A mice in bioassays. They say that the mice develop too many tumors spontaneously, and that any tumors caused by the chemical are tumors that were already on the way. You never get 100 tumors in control animals, never.
Controls are more likely to develop one tumor per three or four mice.
Animals exposed to a carcinogen, on the other hand, can develop up to 200 tumors per lung. So you really do induce tumors in this model. Critics also note that the type of tumor that develops in Strain A mice is different than the kind of lung tumors that develop in humans. True, said Stoner. This includes both women who smoke and those who do not smoke.