Gene therapy is risky - the death was due to cancer - and it's used on patients that can survive without it but have virtually no quality of life. Jesse's death was a setback because the chances of cancer occurring were initially thought to be fairly low. We've since learned otherwise.
The problem is that gene therapies in humans are unable to use targeted insertions (e.g. full gene replacement through recombination). Instead, we're forced to use viral vectors, typically adenoviruses, that insert randomly. Then we hope that it fully inserted the target gene and it's regulating sequences (if necessary) - the cells that test out positive are re-implanted in the patient. Trouble is, that insertion may have disrupted another gene, turning it from protooncogene into full oncogene, from where it causes cancer. All patients who received gene therapy right now - limited primarily to Severe Combined Immunodeficiency (SCIDs), thalassemias, and other severe blood/immune disorders because of the risks - have a risk of developing cancer.
Once someone works out the vector/insertion problem to get reliable results with a targeted insertion, gene therapy will be a lot less risky because the chances of insertion in a protooncogene will be more or less eliminated.
Regardless, this is good news. Thalassemias are nasty.
