The Application segment of the Enzyme Replacement Therapy (ERT) Drugs market is organized around four lysosomal storage disorders: Gaucher disease, Fabry disease, Pompe disease, and mucopolysaccharidoses (MPS). Gaucher disease accounts for the largest share, driven by early diagnostic uptake and multiple approved products, contributing roughly 35 % of total market revenue. Fabry disease follows with an estimated 30 % share, reflecting growing prevalence awareness and expanding therapeutic pipelines. Pompe disease represents about 20 % of the market, supported by recent approvals for infantile‑onset and late‑onset forms. The remaining 15 % is attributed to the broader MPS category, which includes several sub‑types each with niche product offerings. Collectively, these application sub‑segments define the market’s overall size, as each disease‑specific demand translates directly into sales volume for ERT drugs. This structure is central to the Enzyme Replacement Therapy (ERT) Drugs segmentation analysis and informs resource allocation across R&D, manufacturing, and commercial strategies.