TYK2 and spondyloarthropathy: • Susceptibility to spondyloarthropathies and related conditions is associated with IL-23 pathway–related gene polymorphisms, including those in the genes encoding IL-23A and IL-23 receptor, JAK2 and TYK2, STAT3, and IL-17RA (76, 79)• HLA-B27, a ubiquitous MHC Class I molecule, is associated with spondyloarthropathies, and approximately 25% of patients with PsA are positive for HLA-B27 (77)• Studies conducted in HLA-B27 transgenic rats indicate possible links between HLA-B27 and misfolding, ER stress, and augmented induction of the IL-23 pathway (80)