Mucopolysaccharidoses (MPS) disorders are a group of ultra-rare, inherited, lysosomal storage diseases caused by enzyme deficiencies that result in accumulation of glycosaminoglycans (GAGs) in cells throughout the body including the brain, typically leading to early death. Current treatments do not address the progressive cognitive impairment observed in patients with neuronopathic MPS disease. The rarity and clinical heterogeneity of these disorders as well as pre-existing brain disease in clinically diagnosed patients make the development of new therapeutics utilizing a traditional regulatory framework extremely challenging. Children with neuronopathic MPS disorders will likely sustain irreversible brain damage if randomized to a placebo or standard-of-care treatment arm that does not address brain disease. The United States Food and Drug Administration (FDA) recognized these challenges, and, in 2020, issued final guidance for industry on slowly progressive, low-prevalence, rare diseases with substrate deposition that result from single enzyme defects, outlining a path for generating evidence of effectiveness to support accelerated approval based on reduction of substrate accumulation [1]. Neuronopathic MPS disorders, which are characterized by the accumulation of the GAG heparan sulfate (HS) in the brain, fit the intended disease characteristics for which this guidance was written, but to date, this guidance has not yet been applied to any therapeutic candidate for MPS. In February 2024, the Reagan-Udall Foundation for the FDA convened a public workshop for representatives from the FDA, patient advocacy groups, clinical and basic science research, and industry to explore a case study of using cerebrospinal fluid (CSF) HS as a relevant biomarker to support accelerated approval of new therapeutics for neuronopathic MPS disorders. This review provides a summary of the MPS presentations at the workshop and perspective on the path forward for neuronopathic MPS disorders., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Joseph Muenzer: Consultant and/or served on advisory boards for Takeda, Sanofi, REGENXBIO, Denali Therapeutics, Inc., and JCR Pharmaceuticals. Carole Ho: Employment and stock ownership with Denali Therapeutics, Inc. Heather Lau: Employment and stock ownership with Ultragenyx Pharmaceutical, Inc. Mark Dant: None. Maria Fuller: Research and travel support from Abeona, BioMarin, Denali Therapeutics, Inc., Paradigm, Sanofi-Genzyme, Takeda (formerly Shire), REGENXBIO, Taysha Gene Therapies, and Ultragenyx. Nidal Boulos: Employment with REGENXBIO. Patricia Dickson: Research support from BioMarin Pharmaceutical Inc., Alnylam, and M6P Therapeutics; consultant for Mandos Health. Matthew Ellinwood: Honoraria, travel, conference registration, and/or consultancy paid to or received by the Society from: ACMG, APHL, Denali Therapeutics, EdiGene Biotechnology USA, Eli Lilly and Company, EveryLife Foundation for Rare Diseases, Global Genes (Rare Drug Dev. Symp.), Guidepoint Global, LLC, PRECISIONadvisors, REGENXBIO, Inc., Terrapin (World Orphan Drug Cong-USA), WORLDSymposium™. Simon Jones: Investigator and consultant for Sanofi, Takeda, Ultragenyx, Orchard Therapeutics, Intrabio, Mina, Prevail, and Denali Therapeutics, Inc.; consultant for Ultragenyx. Eric Zanelli: Shareholder of Allievex. Cara O'Neill: Consultant and/or serve on advisory councils for BioMarin Pharmaceutical, Denali Therapeutics, Inc., JCR Pharmaceuticals, Novel Pharma, and Ultragenyx., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)