You begin with a condition, and you have no idea what sort of a gene causes it. So you map the gene's location on a chromosome and clone it, and then you find out what it does. Quite often, the function of the gene is very surprising. With the gene identified, further study revealed a number of mutations that disabled it. A gene that leads to cancer when it is disabled is known as a tumor suppressor gene.
Tumor suppressor genes work by producing a protein that helps prevent run-away cell division.
Initially, de la Chapelle and the other members of the research team had no idea what kind of protein the undamaged PJS gene produced, or what role that protein played in the cell. But by comparing the structure of the PJS gene with already-known genes, they were startled to find that the PJS gene produces a type of enzyme known as a protein kinase. As far as we know, this is the first time a protein kinase gene has been identified as a tumor suppressor gene. Cells have genes for many types of protein kinase enzymes, and some of these enzymes do play a role in causing cancer. But in those cases, cancer rises because the genes produce too much of the enzyme.