PRSS57 (neutrophil serine protease 4, NSP4) is a serine protease with distinctive substrate specificity and emerging roles in multiple biological contexts. The enzyme preferentially cleaves substrates after arginine residues 12, with the unique ability to also process post-translationally modified arginine forms including citrulline and methylarginine 3. Structurally, NSP57 employs an uncommon recognition mechanism featuring a fully occluded S1 pocket and stabilization of substrate P1-arginine in a noncanonical "up" conformation, distinguishing it from canonical trypsin-like proteases 3. PRSS57 is expressed in hematopoietic stem and progenitor cells, where it may participate in intercellular communication within the bone marrow stem cell niche through interaction with inhibitor SPINK2 4. Recent genetic studies implicate PRSS57 in rheumatoid arthritis pathogenesis, identifying it as a potential risk factor through plasma proteome association 5, and in lipid metabolism regulation relevant to cardiovascular health 6. Additionally, PRSS57 expression shows age-dependent variation in spermatozoa, correlating with declining male fertility in advancing age 7. While biological functions in immune regulation and reproduction are suggested, the clinical significance of PRSS57 requires further functional characterization to establish therapeutic potential.