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To determine cellular distribution of cell adhesion molecule 1 (CADM1), an immunoglobulin superfamily member, in the human oxyntic gastric mucosa, and to explore possible involvement in the development and peritoneal dissemination of signet ring cell (SRC) gastric carcinoma, which often develops in the oxyntic mucosa.Immunohistochemistry and double immunofluorescence were conducted on surgical specimens of normal and SRC-bearing stomachs and peritoneal metastatic foci of SRCs. KATO-III (lacking CADM1) and HSC-43 (expressing CADM1) SRC cell lines were cocultured on a Met-5A mesothelial or TIG-1 fibroblastic cell monolayer.In the oxyntic gland, some neck and nearly all base glandular cells were CADM1-positive, and mucin 5AC-positive cells were CADM1-negative, while some mucin 6-positive neck cells were CADM1-positive. Foveolar-epithelial, parietal, and endocrine cells were CADM1-negative. CADM1 was negative in all SRC carcinomas that were confined within the submucosa (n = 11) and all but one of those invading deeper (n = 15). In contrast, peritoneal metastatic foci of SRCs were CADM1-positive in five out of eleven cases (P < 0.01). In the cocultures, exogenous CADM1 made KATO-III cells adhere more and grow faster on a Met-5A monolayer, not on TIG-1 monolayers. HSC-43 cells adhered more and grew faster on Met-5A than on TIG-1 monolayers, which were partly counteracted by a function-neutralizing anti-CADM1 antibody.Nearly all chief cells and a part of mucous neck cells express CADM1. SRC gastric carcinoma appears to emerge as a CADM1-negative tumor, but CADM1 may help SRCs develop peritoneal dissemination through promoting their adhesion and growth in the serosal tissue.