Horvitz's later research continued to use "C. elegans" to analyze the genetic control of animal development and behavior, as well as to link discoveries in the nematode to human diseases, particularly cancer and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). He made further advancements in defining the molecular pathway of programmed cell death, and has identified several key components, including: EGL-1, a protein which activates apoptosis by inhibiting CED-9; transcription factors "ces-1" and "ces-2," and "ced-8," which controls the timing of cell death. He has also continued work on heterochronic mutants and other aspects of cell lineage, and established lines of research in signal transduction, morphogenesis, and neural development. Horvitz has collaborated with Victor Ambros and David Bartel on a project to characterize the complete set of the more than 100 microRNAs in the "C. elegans" genome.