Patients and their siblings, as well as any other willing family member (affected or non-affected) are asked to have a blood sample (EDTA tube for genetic analysis and a serum sample for Helicobacter pylori status) taken by their physician. Genomic DNA is extracted from lymphocytes and a genome-wide scan is performed using a standard marker set. A computer program is used to verify sibling relationships. Individuals not found to be full siblings are excluded from subsequent analyses. Maximum likelihood score (MLS) and the nonparametric linkage score (NPL) is used to estimate the degree of linkage.