ST14 and influenza: The endogenous regulator serine protease inhibitor Kazal‐type 6 (SPINK6) inhibits HAT/KLK5 to restrict influenza virion maturation (208), while dichlorobiphenyl-containing matriptase inhibitors achieve ultrahigh potency (Ki < 3 nM) through chemical optimization, demonstrating exceptional thrombin selectivity and concentration-dependent inhibition of H9N2 viral replication in MDCK(II) cells (209).