At the heart of this search for new cancer drugs are patients like Joe Bellino, who was a post office clerk until his cancer made him too sick to work. Seven years ago, he went into the hospital for hernia surgery, only to learn he had liposarcoma, a rare cancer of fat cells. A large tumor was wrapped around a cord that connects the testicle to the abdomen. “I was shocked,” he said in an interview this summer.

Companies have long ignored liposarcoma, seeing no market for drugs to treat a cancer that strikes so few. But it is ideal for testing Sanofi’s drug because the tumors nearly always have the exact genetic problem the drug was meant to attack — a fusion of two large proteins. If the drug works, it should bring these raging cancers to a halt. Then Sanofi would test the drug on a broad range of cancers with a similar genetic alteration. But if the drug fails against liposarcoma, Sanofi will reluctantly admit defeat.

“For us, this is a go/no-go situation,” said Laurent Debussche, a Sanofi scientist who leads the company’s research on the drug.

The genetic alteration the drug targets has tantalized researchers for decades. Normal healthy cells have a mechanism that tells them to die if their DNA is too badly damaged to repair. Cancer cells have grotesquely damaged DNA, so ordinarily they would self-destruct. A protein known as p53 that Dr. Gary Gilliland of Merck calls the cell’s angel of death normally sets things in motion. But cancer cells disable p53, either directly, with a mutation, or indirectly, by attaching the p53 protein to another cellular protein that blocks it. The dream of cancer researchers has long been to reanimate p53 in cancer cells so they will die on their own.

Image Joe Bellino, shown with his daughter, had a rare kind of cancer that could respond well.

The p53 story began in earnest about 20 years ago. Excitement ran so high that, in 1993, Science magazine anointed it Molecule of the Year and put it on the cover. An editorial held out the possibility of “a cure of a terrible killer in the not too distant future.”