Guillemin has been studying the involvement of tryptophan catabolism (via the kynurenine pathway- KP B) in human neurodegenerative diseases for more than 20 years.Guillemin and his teams have conducted research that demonstrates the importance of the KP in multiple sclerosis, Alzheimer's disease and motor neuron disease, which has diagnostic, prognostic and therapeutic potential.This research has been extended to looking at the chemicals in the brain and their relation to other diseases such as depression, suicide, autism and cancers.