Immunodeficient animals play an important role in biomedical research by allowing the examination of engrafted human cells under in vivo conditions, without risk of rejection. Researchers use these animals to study basic biology, model human diseases, and develop new therapies. Many fields have benefitted from this type of research, including stem cell biology, regenerative medicine, transplantation, infectious diseases, immunology, and cancer.

For more than 50 years, scientists have used the roundworm Caenorhabditis elegans (C. elegans)—a hermaphroditic nematode that is widely recognized as an important tool for genetics, cell biology, and neuroscience research. The entire adult nervous system and cell lineage of C. elegans was mapped in 19861 and since then, scientists have used this map of neuron connectivity to help elucidate diverse neurobiological mechanisms. Compared to the 100 billion neurons in the human brain, C.