HLA-DRB1 and psoriatic arthritis: The effects of tryptophan at HLA-DRB1 position 9 and lysine at position 71 were robust to potential confounders, including immunosuppressant cotherapy, threshold of ADA detection, genotype of the psoriasis susceptibility allele HLA-C*06:02, sample timing, comorbidities (including psoriatic arthritis and diabetes), age, age of psoriasis onset, sex, and baseline disease severity (see Supplemental Table 6).