Filaggrin is initially synthesized as a large, insoluble, highly phosphorylated precursor containing many tandem copies of 324 residues. This precursor is dephosphorylated and proteolytically cleaved by several proteases, including the undefined protease PEP1 (Resing et al. 1996), mu-calpain (Yamazaki et al. 1997), furin, PCSK6 (PACE4) (Pearton et al. 2001), PRSS8 (cap1) (Leyvraz et al. 2005), ST14 (matriptase) (List et al. 2003), CELA2 (Bonnart et al. 2010), CASP14 (Denecker et al. 2007) and Kallikrein-related peptidase 5 (KLK5) (Sakabe et al. 2013). Filaggrin is further processed and proteolytically degraded by CASP14 (Eckhart & Tschachler 2011).