["Crohn's disease (CD) is a chronic inflammatory bowel disease (IBD) of unknown etiology. We aimed to identify the etiological agent of CD using a molecular cloning strategy that was particularly focused on identifying agents causing immune abnormalities and infectious agents.", 'We constructed a cDNA library derived from the inflamed intestinal tissue of a CD patient, and screened 1.5 million clones in this library with the serum from another typical CD patient. The expressed cDNA clones that positively reacted with the serum were then expressed as fusion proteins with glutathione S-transferase, and western blotting was performed using the sera of 22 CD, 13 ulcerative colitis (UC), and 16 non-IBD patients.', "We identified nine positive clones that did not contain any viral or bacterial genomic DNA. Of these, we selected one clone (clone 50) with which the typical CD patient's serum most strongly reacted. Clone 50 is highly homologous to the antioxidant protein peroxiredoxin 6. In western blotting, the sera of 47.6% CD patients (small intestine type 80%, large and small intestine type 43%, large intestine type 0%) showed strong reactivity to clone 50, none of the UC patients were reactive to clone 50, and 18.8% of non-IBD patients were very weakly reactive to it. We also found that the expression of peroxiredoxin 6 was significantly increased in inflamed intestinal epithelia of CD."] Does molecular cloning reveal nearly half of patients with Crohn 's disease have an antibody to peroxiredoxin 6-like protein?