AIM: To explore dysregulation of c-fos in several human malignancies, and to further investigate the role of c-fos in Helicobacter pylori ( H. pylori)-induced gastric precancerosis.METHODS: Four-week-old male Mongolian gerbils were H. pyloriNCTC 11 637 in Brucella broth were inoculated orally into 20 Mongolian gerbils. Another 20 gerbils were inoculated with Brucella broth as controls. 10 of the infected gerbils and 10 of the non- infected control gerbils were sacrificed at 25 and 45 weeks after infection. The stomach of each gerbil was removed and opened for macroscopic observation. The expression of c-fos was analyzed by RTPCR and immunohistochemical studies in H. pylori-induced gastric precancerosis of Mongolian gerbil. Half of each gastric antrum mucosa was dissected for RNA isolation and RTPCR. β-actin was used as the housekeeping gene and amplified with c-fos as contrast. PCR products of c-fos were analyzed by gel image system and the level of c-fos was reflected with the ratio of c-fos/β-actin. The immunostaining for c-foswas conducted using monoclonal antibody of c-fosand the StreptAvidin-Biotin-enzyme Complex kit.RESULTS: H. pyloriwas constantly found in all infected animals in this study. After infection of H. Pylorifor 25 weeks,ulcers were observed in the antral and the body of stomach of 60 % infected animals (6/10). Histological examination showed that all animals developed severe inflammation, especially in the area close to ulcers, and multifocal lymphoid follicles appeared in the lamina propria and submucosa. After infection of H. Pylorifor 45 weeks, severe atrophic gastritis in all infected animals, intestinal metaplasia in 80 % infected animals (8/10) and dysplasia in 60 % infected animals (6/10) could be observed. C-fos mRNA levels were significantlyhigher after infection of H. pylorifor 25 weeks (1.84±0.79),and for 45 weeks (1.59±0.37) than those in control-animals (0.74±0.22, P＜0.01). C-fos mRNA levels were increased 2.5-fold by 25th week (P＜0.01) and 2.1-fold by 45th week (P＜0.01) in precancerosis induced by H. pylori, when compared with normal gastric epithelium of Mongolian gerbil. Immunohistochemical staining revealed exclusive nuclear staining of c-fos. Furthermore, there was a sequential increase in c-fos positive cells from normal epithelium to precancerosis.CONCLUSION: The study suggested that overexpression of c-fos occurs relatively early in gastric tumorigenesis in this precancerosis model induced by H, pylori.