A defined piece of single-stranded nucleic acid which binds to and initiates replication of a target nucleic acid template in an assay intended to be used to evaluate a clinical specimen to diagnose, monitor or predict the inherited genetic condition ankylosing spondylitis which is associated with the HLA-B27 allele and/or other genetic markers including polymorphisms of the CYP2D6 gene or IL-1 gene complex.