Cystatin-B (CSTB)

Cystatin-B (CSTB) is a small (11 kDa) intracellular thiol proteinase inhibitor belonging to the type II cystatin family[1][5]. CSTB inhibits several lysosomal cysteine proteases (notably cathepsins L, H, and B), thereby protecting cells against inappropriate or excessive proteolysis, especially from lysosomal leakage[1][5]. In neurons, CSTB localizes to the cytoplasm, nucleus, and synaptic compartment, where it contributes to cell cycle regulation, apoptosis inhibition, and synaptic plasticity[1][3]. Genetic deficiency of CSTB results in progressive myoclonic epilepsy type 1 (EPM1), a severe neurodegenerative disorder[3]. CSTB is overexpressed in multiple cancers and can serve as a diagnostic or prognostic biomarker in ovarian and cervical cancer[2][4]. In veterinary medicine, urinary CSTB is an emerging biomarker of acute kidney injury[6]. CSTB is not a typical therapeutic target (not a receptor, enzyme, or channel), but its biological roles and disease associations make it highly relevant for pathophysiology and biomarker research.

Other names

Stefin BSTFBPMECPI-BLiver thiol proteinase inhibitorEpilepsy, progressive myoclonic 1EPM1EPM1AULDepididymis secretory sperm binding proteinCST6 (frequently confused but CSTB and CST6 are different genes; CST6 is cystatin E/M)

Not applicable for direct drug targeting. The molecule acts via inhibition of cysteine proteases. No approved therapeutics acting *on* cystatin B, but it may be a downstream or mechanistic biomarker for therapies affecting protease pathways.