In his early work at MIT, Horvitz continued his work on cell lineage and cell fate, using "C. elegans" to investigate whether there was a genetic program controlling cell death, or apoptosis. In 1986, he identified the first "death genes", "ced-3" and "ced-4". He showed that functional "ced-3" and "ced-4" genes were a prerequisite for cell death to be executed. He went on to show that another gene, "ced-9", protects against cell death by interacting with "ced-4" and "ced-3", as well as identifying a number of genes that direct how a dead cell is eliminated. Horvitz showed that the human genome contains a "ced-3"-like gene.