While key cytokines and driving mechanisms are shared between SLE and psoriasis, the two conditions also have some distinct underlying pathophysiological mechanisms.4 5 In addition, shared loci conferring genetic susceptibility to the two conditions, including tumour necrosis factor (TNF) receptor-associated factor 3 interacting protein 2,6 7 signal transducer and activator of transcription 4 (STAT4)8 9 and protein tyrosine phosphatase non-receptor type 22, have been reported.10 11 Despite dissimilar aspects of their pathogenesis, psoriasis and SLE may have phenotypical similarities.12 This evidence concerns the gene PTPN22 and systemic lupus erythematosus.