Two developed the disease because IL2RG inserted itself into the cellular genome next to a known cancer-causing gene and activated it, but the cause of the third cancer case had not been found.
One notorious example came when just about every major drug company in the mid-1990s rushed to develop drugs to block a cancer-causing gene called ras.
It turned out that the virus used to deliver the gene therapy was also inserting itself into cancer-causing areas of the genome, and not just where it was needed to cure the kids.
In the past three months scientists have unearthed solid evidence for six diabetes-causing gene variants, several variants involved in prostate cancer and an obesity gene that adds 7 pounds of fat if you have the bad version.