Interestingly from a citrullination point of view, a human genetic variant of PTPN22 (Protein Tyrosine Phosphatase Non-Receptor Type 22) is associated with severe forms of alopecia areata (Kemp et al., 2006; Betz et al., 2008), a finding which was further corroborated by two independent genome-wide association studies, which identified susceptibility loci in PTPN22 in alopecia areata patients (Martinez-Mir et al., 2007; Petukhova et al., 2010). This evidence concerns the gene PTPN22 and alopecia areata.