Peptidylarginine deiminase (PAD) (EC 188.8.131.52) catalyzes deimination of arginine residues in proteins in a calcium-ion-dependent manner , generating citrulline residues . The deiminated proteins show lowered isoelectric points, affecting protein interactions dependent on ionic charge. Stability of hydrogen bonds may also be disrupted. Substrate proteins of PAD in the epidermis include keratin K 1 and filaggrin, but not loricrin. Because keratin K 1 is assumed to be associated with filaggrin through ionic zipper interaction and with loricrin through velcro interaction, respectively, alteration of PAD-dependent protein deimination would have a significant biological effect on terminal differentiation of keratinocytes. Psoriatic hyperproliferative epidermis is characterized by decreased deiminated proteins, especially keratin K 1. Whether the finding is associated with a defect of PAD in the psoriatic hyperproliferative epidermis remains to be determined.