Jesse Gelsinger was 18 and healthy when he died in 1999 during a gene-therapy experiment. He had a condition called ornithine transcarbamylase deficiency (OTC), but it was under control through a combination of diet and medication. Like others with the disorder, Gelsinger lacked a functional enzyme involved in breaking down ammonia, a waste product of protein metabolism that becomes toxic when its levels become too high. The gene therapy that he received used a viral vector to introduce a normal gene for the enzyme.