GMPPB (GDP-mannose pyrophosphorylase B) functions as the catalytic subunit of the GMPPA-GMPPB mannose-1-phosphate guanylyltransferase complex, catalyzing the formation of GDP-mannose, an essential precursor for glycan moieties of glycoproteins and glycolipids 1. The protein regulates GDP-alpha-D-mannose levels together with GMPPA and can catalyze the reverse reaction in vitro 1. GMPPB mutations are associated with muscular dystrophy-dystroglycanopathies, which are characterized by deficient glycosylation of alpha-dystroglycan 2. Clinical presentations include congenital myasthenic syndromes and limb-girdle muscular dystrophies 34. In a large Chinese cohort, GMPPB mutations were identified among the common causes of dystroglycanopathy 2. The gene has also been implicated in broader disease contexts, with proteomic studies identifying GMPPB as a potential therapeutic target for glioblastoma 5 and showing causal connections to ADHD risk 6. Additionally, GMPPB-related disorders fall within metabolic cardiomyopathies linked to carbohydrate metabolism defects 7, highlighting its critical role in cellular glycosylation processes essential for multiple organ systems.