PTPN22 and psoriatic arthritis: Others, however, found that C1858T substitution is only associated with higher susceptibility of psoriatic arthritis, but not of PSO itself [267,268,269], whereas again others did not find any significant association between PTPN22 and PSO [270,271,272,273,274], leaving the putative role of PTPN22 dysfunction in PSO rather controversial.