**1. Introduction**

Peptidylarginine deiminases (PADIs) are a group of enzyme that converts peptidyl-arginine to peptidyl-citrulline, also called protein citrullination, in the presence of Ca2<sup>+</sup> [1]. There are five members in the human PADI family, and each member has its own tissue distribution and substrate specificity [2]. PADIs and protein citrullination is known not only to contribute to the pathogenesis of several autoimmune diseases, such as rheumatoid arthritis (RA) and multiple sclerosis [1,3,4], but they were also recently found to facilitate cancer invasion and metastasis [5,6]. In human leukocytes, PADI2 and PADI4 are highly expressed [1]. Our previous study showed that the expression of PADI2 and PADI4 was remarkably increased during macrophage differentiation whereas the addition of lipopolysaccharides (LPS) increased the levels of citrullinated proteins. We further provided the evidence that PADI2 might play a critical role in the inflammatory response using plasmid-encoding short hairpin RNA-targeting PADI2 [7]. This result is consistent with that reported by Bawadekar et al., which demonstrated that PADI2-deficient mice showed a reduced joint inflammation in murine tumor necrosis factor-alpha (TNF-α)-induced arthritis [8].

Recently, clustered regularly interspaced short palindromic repeats (CRISPR) and the CRISPR-associated protein-9 nuclease (Cas9) system (CRISPR–Cas9 system), an effective way to edit genome [9], has become a powerful tool for investigating the biologic function of a specific gene [10]. We hypothesized that PADI2 is required for multiple domains of macrophage functions. Therefore, we used the CRISPR–Cas9 system to knockout PADI2 in macrophages and to evaluate the effects of PADI2 knockout on various functions of macrophages, including inflammation, cell survival, and adhesion capacity.
