Swiftly maneuvering through the blood stream, Roxxi, an RX5-E Class nanobot, spots a medulloblastoma cell. Roxxi pulls out a chemoblaster and lets loose with several rounds of green projectiles that seek and destroy the malignant cell before it can multiply.

Although it's a shoot-em-up video game - complete with a "Teen" rating - Re-Mission, the game that features Roxxi, is geared toward a very specific audience: teenagers and young adults being treated for cancer. Roxxi relies on an unconventional combat arsenal to battle bacterial infections, constipation, anxiety, and other treatment-related side effects.

Re-Mission, explained Dr. Steve W. Cole, vice president of research for HopeLab, the nonprofit organization behind the game, was designed to help young cancer patients better understand their cancer "and to do it in a way that helps them understand how their behavior impacts the biology of their disease."

In addition to offering a welcome distraction from treatment and side effects, the game reinforces the importance of things like sticking to a home chemotherapy regimen or taking a stool softener in the event of constipation - something that, left untreated, can be extremely dangerous, even fatal.

And it appears to work.

HopeLab - at the direction of its founder, Pam Omidyar, who worked as a research assistant in an immunology lab while pursuing a master's degree in molecular biology - put the game through the rigors of a randomized, controlled intervention trial. Involving 375 male and female patients aged 13 to 29, the trial demonstrated that, compared with patients who regularly played another popular video game, patients who played Re-Mission had better scores on psychosocial assessments such as quality of life. More important, though, patients who played Re-Mission had significantly higher chemotherapy blood metabolite levels and rates of antibiotic utilization.

"The upshot is that this game works," Dr. Cole said. "It shows that video games can be a powerful tool for behavior change in the real lives of patients."

Re-Mission was carefully developed to ensure that its presentation of different cancers was biologically precise, while at the same time ensuring that it had the elements of fun and challenge of popular video games. HopeLab recruited cancer researchers, doctors, nurses, and psychologists to work with experienced video game developers to craft Re-Mission's 20 stages, and HopeLab solicited feedback from and regularly tested the game with young cancer patients to make sure that it was fun, and above all, spoke to their situations.

"We have heard over and over from kids who played this game that ‘This is about me and the issues I'm confronting,'" Dr. Cole says. "It's gratifying to hear them say that we hit the mark."

Re-Mission even has its own Internet community, http://www.re-mission.net, where young cancer patients, clinicians, and caregivers can order the game, free of charge. The Web site also has information about cancer and message boards and blogs.