For many years, Parkinson's disease and dementia with Lewy bodies have been defined using clinical features. With recent advances in biomarkers, particularly the alpha-synuclein seed amplification assay that allows us to detect misfolded and aggregated alpha-synuclein in cerebrospinal fluid, it is now possible to identify the presence of pathological neuronal alpha-synuclein in living patients. With input from diverse stakeholders, a new biological definition of neuronal alpha-synuclein disease (NSD) has been proposed, along with an integrated staging system (NSD-ISS). In this interview, we discuss the development of this biologic definition for Parkinson’s disease and the staging system with Dr. Lana Chanine, one of the lead authors of the recently published manuscript in The Lancet Neurology that describes this work. It is important to note that the NSD and NSD-ISS are for research purposes only right now, and they are not ready to be implemented in the clinic. Lana is a Movement Disorders Neurologist and Associate Professor of Neurology at the University of Pittsburgh. This podcast is geared toward researchers and clinicians. If you live with Parkinson’s or have a friend or family member with PD, listen to The Michael J. Fox Foundation Parkinson’s Podcast. Hear from scientists, doctors and people with Parkinson's on different aspects of life with the disease as well as research toward treatment breakthroughs at https://www.michaeljfox.org/podcasts.