Our results reveal that genetic background significantly influenced the development of pituitary (Fig. 1 and Table 1), glucagon-expressing pancreatic (Fig. 3), adrenal and gonadal tumours (Fig. 1 and Table 1) in Men1+/- mice, which represent a model for the MEN1 syndrome, thereby supporting the role for genetic modifiers in altering the phenotype of this autosomal dominant disorder. The gene discussed is GCG; the disease is Down syndrome.