The post-translational conversion of peptidylarginine to peptidylcitrulline, a 
process also known as citrullination, is catalyzed by the enzyme family of 
peptidylarginine deiminases (PADs) and has been demonstrated to be involved in 
many physiological processes, including the regulation of gene expression. In 
addition, citrullination has been shown to be associated with several diseases, 
such as cancer, multiple sclerosis, rheumatoid arthritis, and Alzheimer's 
disease. To get more insight into the role of PAD enzymes and citrullination in 
both health and disease, experimental strategies to study PAD activity and to 
characterize citrullinated proteins in complex biological samples are crucial. 
Here, we describe the chemical, proteomic and antibody-based procedures that are 
currently available and discuss their applicability for the analysis of complex 
samples. The methods that have been developed can be used to provide more 
insight in the substrate specificity of PAD enzymes. Because the evidence that 
PADs play a pathophysiological role in the diseases mentioned above is 
increasing, they become attractive targets for therapeutic interventions. More 
knowledge of PAD specificity and the availability of reliable, high-throughput 
assays for PAD activity will facilitate the development of highly specific PAD 
inhibitors.