Former U.S. President Jimmy Carter appeared relaxed and even made the occasional joke when he publicly announced yesterday that he has melanoma that has spread to his liver and brain. The 90-year-old expressed hope that he’d benefit, as many are these days, from cutting-edge therapies that help the immune system destroy cancer cells. (These treatments topped Science’s Breakthroughs of the Year in 2013.) Melanoma, in some patients, was among the first cancers to succumb to one of these immunotherapies, and the specific treatment Carter is taking has many oncologists excited: It’s a monoclonal antibody called pembrolizumab and sold under the brand name Keytruda. Approved in the United States a year ago for advanced melanoma, pembrolizumab belongs to a hot class of drugs called PD-1 inhibitors. By blocking the PD-1 protein, the therapy allows the body to make T cells that can chase after a cancer.

Still, researchers have a lot to learn about pembrolizumab and other PD-1 therapies in development. Keytruda is also extremely expensive, at about $150,000 a year. In clinical trials, PD-1 blockers generally work in less than half the participants (see here and here). Research published earlier this year suggested PD-1 inhibitors may work best on tumors with lots of mutations, and a small clinical trial of pembrolizumab backed this up. It found that people with advanced cancer were far more likely to respond if they had so-called mismatch repair mutations in their tumors. This could also help explain why, so far, PD-1 inhibitors have produced the best outcomes in people with lung cancer and melanoma—both are often mutation-heavy tumors.

What all this means for individual cancer patients such as Carter is still uncertain. “I’ve had a wonderful life,” he said in a televised news conference from the Carter Center in Atlanta, smiling broadly. “I’ll be prepared for anything that comes.”