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Sierra Ramos, Rafaela, Nubia Muñoz Calero, Salvador Peña, I. Biemond, W. van Duijn, C. B. H. W. Lamers, Sibylle Teuchmann, S. Hernández, och P. Correa. Antibodies to Helicobacter Pylori and Pepsinogen Levels in Children From Costa Rica: Comparison of Two Areas With Different Risks for Stomach Cáncer. 1992.

Sierra Ramos, Rafaela, et al. Antibodies to Helicobacter Pylori and Pepsinogen Levels in Children From Costa Rica: Comparison of Two Areas With Different Risks for Stomach Cáncer. 1992.