In the multivariate analysis, male gender (p < 0.001), younger age (p < 0.001), a disease duration of more than 3 years (p < 0.001), a family history of SpA (p < 0.001), uveitis (p < 0.001), ASDAS-CRP (p < 0.001), and biologic treatment (p < 0.001) were the main variables that were independently related to HLA-B27 presence, whereas a diagnosis delay time >36 months (p < 0.001) and psoriasis (p < 0.001) were independently related to HLA-B27 absence. The gene discussed is CRP; the disease is psoriasis.