Posttranslational modifications are chemical changes to proteins that take place 
after synthesis. One such modification, peptidylarginine to peptidylcitrulline 
conversion, catalysed by peptidylarginine deiminases, has recently received 
significant interest in biomedicine. Introduction of citrulline dramatically 
changes the structure and function of proteins. It has been implicated in 
several physiological and pathological processes. Physiological processes 
include epithelial terminal differentiation, gene expression regulation, and 
apoptosis. Rheumatoid arthritis, multiple sclerosis, and Alzheimer's disease are 
examples of human diseases where protein citrullination involvement has been 
demonstrated. In this review, we discuss our current understanding on the 
importance of protein deimination in these processes. We describe the enzymes 
catalyzing the reaction, as well as their known protein substrates. We review 
the citrullinated peptide epitopes that are proposed as disease markers, 
specifically recognized in certain human autoimmune disorders. The potential 
autopathogenic role of citrullinated epitopes is also discussed.