Peptidylarginine deiminases (PAD) are a group of post-translational modification 
enzymes that citrullinate (deiminate) protein arginine residues in a calcium 
ion-dependent manner. Enzymatic citrullination abolishes positive charges of 
native protein molecules, inevitably causing significant alterations in their 
structure and functions. Citrullinated protein has an important physiological 
purpose; the formation of a cornified layer of skin that covers the human body. 
Despite this beneficial function, citrullinated protein also has a negative 
side, because this protein's accumulation in the brain is a possible cause of 
Alzheimer's disease. In the present review, we introduce PAD and their protein 
citrullination function, now considered critical for advancing research on aging 
and disease.