Michael Feeney is nine years old. Since the age of six, Michael has been battling a type of bone cancer called Ewing's sarcoma. Though he is enrolled in an experimental trial right now, the Feeney family has an unusual Plan B if his current treatment does not work out. There is a mouse with Feeney's tumor growing in its body right now, and doctors have found a cocktail of drugs to that has shrunk it. If the trial is unsuccessful, the Feeney's have spent over $25,000 on what they call a "home run".

Mice have been used in laboratory experiments for years and even the concept of implanting mice with human tumors is not a new one. But techniques have gotten better and, for the first time, companies are commercializing the process used in drug development and medical treatment. They call the mice "avatars".

The process goes like this: doctors remove the tumor, or a chunk of it, and send it to a laboratory (in the Feeneys' case, it was Champions Oncology in New York City). Scientists cut the tumor into five pieces, slice the skin of an anesthetized mouse and place it underneath the skin. The slicing and placement take only five minutes. Two months later, the first mouse's transplant is removed, cut into pieces, and each new piece is placed into a new mouse. After another month, there are enough mice available for testing.

Two different studies have been published in which researchers gave mice a form of cancer and tweaked the treatment according to how the animals responded. The mice can also be, and have been, used to treat illnesses other than cancer. Human immune systems were grown in mice to study type 1 diabetes. Another study implanted intestinal bacteria from humans into mice.

Critics are numerous and have a lot to say. The total length of the procedure is long and patients have died waiting for their mice to be done growing their tumors. Sometimes the tumors do not grow at all. Mice can die during the transplant or when undergoing treatment. The tumor may not behave the same in mice. Treatments that are effective in mice may not have the same effect in humans. Critics are unsure whether it will prolong patients' lives and the process is very financially draining; it can cost upwards of tens of thousands of dollars, which insurance does not cover. Skeptics say that a randomized trial would need to occur in order to prove that people with avatars would do better than more traditional methods.

But for some cancer patients and doctors, who have exhausted other methods, the risks are worth it. Nir Toeb, an Israeli filmmaker who suffered from lung cancer, had 10 tumors in his right kidney. After tests on his avatar, Toeb now has none.

While patients joke about their avatars, they do not feel any personal attachment for them. Mrs. Feeney, Michael's mother, says that, if they need to use the treatment developed on the mice and it works, she will feel forever indebted to the creatures. But others are less sentimental. According to one 60-year-old New Jersey man with lung cancer, "[from] my perspective, the more that die, the better for me."