They have, at least in mice. This article is from 3 years ago, but I haven't heard anything else about this discovery since:
A cancer-proof mouse, which can survive being injected with any number of cancer cells, has been discovered by US scientists. The discovery of the resistant mouse could pave the way for future gene or drug therapies if the mechanism by which it fights cancer can be understoodResearchers at the Wake Forest University School of Medicine in North Carolina have now bred a colony of 700 cancer-proof mice from the resistant male they stumbled across while doing other experiments.
Doctors have known for many years that in rare cases, cancers can mysteriously clear up of their own accord. But this is the first time such cancer-killing ability has been shown to be genetic.
"The power of this resistance seems to be unlimited," said biochemist Zheng Cui, the study leader. "You can give them many, many tumour cells and the mice get rid of them."
"This is at a preliminary stage, but very promising," adds pathologist Mark Willingham, another member of the team. "Our hope is that, some day, this will have an impact on human cancer." ...
awesome, thank you so much :)