“…With regard to the controls source, nine studies [9, 20, 23-27, 31, 34] enrolled hospital-based patients, two studies [29,30] were population-based, and five studies [21,22,28,32,33] enrolled controls of unknown origin. In most of the considered studies, the ratio of patients affected by different psoriasis subtypes was not reported [9,20,21,24,31,32,34], while only 5 and 4 studies were conducted in groups of patients affected predominantly by psoriasis vulgaris [22,25,26,28,33] and psoriatic arthritis [23,27,29,30], respectively. In all studies the used genotyping method was polymerase chain reaction (PCR).…”