CTSA encodes cathepsin A (also called protective protein/carboxypeptidase A), a lysosomal enzyme with dual protective and catalytic functions 1. As a protective protein, CTSA is essential for stabilizing and maintaining the activity of β-galactosidase and neuraminidase 1, forming a functional complex within lysosomes 1. Beyond its chaperone role, CTSA functions as a serine-type carboxypeptidase capable of processing tachykinins and other substrates 2. Pathogenic CTSA mutations cause galactosialidosis, a lysosomal storage disorder characterized by secondary deficiency of its binding partners, leading to severe metabolic dysfunction 1. The disease presents in infantile, late infantile, and juvenile forms with variable severity depending on mutation type 1. Beyond classical galactosialidosis, CTSA dysfunction contributes to broader pathological processes: extracellular vesicle-derived CTSA from obese adipose tissue promotes pancreatic cancer progression and immunosuppression through pseudouridine generation 3, while CTSA mutations are implicated in cerebral small-vessel disease with white matter lesions and psychiatric manifestations 4. CTSA also localizes to multiple cellular compartments including the endoplasmic reticulum, azurophil granules, and extracellular exosomes, expanding its functional significance beyond canonical lysosomal roles 2.