Methylation of histones by protein arginine methyltransferases (PRMTs), in general, is required for mammalian development and plays an important and dynamic role in gene regulation. Protein-arginine deiminases (PADIs) catalyse the deimination of L-arginine residues (L-Arg) in proteins to L-citrulline (L-Cit), thus playing a role in the regulation of development (Guerrin et al. 2003, Ishigami et al. 2002, Kanno et al. 2000, Wang et al. 2004, Nakayama-Hamada et al. 2005, Chavanas et al. 2004).