Autoantibodies against aromatic L-amino acid decarboxylase (Rorsman et al, 1995), histidine decarboxylase (Skoldberg et al, 2003) and cysteine sulfinate decarboxylase (Skoldberg et al, 2004) are all found in patients with APS type I. We have now shown that another enzyme related to PLP is an autoantigen, making PLP a common denominator for enzymes targeted by the immune system in endocrine and nervous system autoimmune diseases. This evidence concerns the gene HDC and autoimmune disease.