SpA, also termed “seronegative” as they do not produce rheumatoid factor nor the anti-citrullinated peptide antibodies (ACPA) observed in RA, represent a group of diseases with common genetic and clinical features, including ankylosing spondylitis (AS), reactive arthritis, psoriatic arthritis (PsA), and SpA associated with inflammatory bowel disease. This evidence concerns the gene PRTN3 and rheumatoid arthritis.