The multiple endocrine neoplasia type 1 (MEN1) gene is a tumor suppressor gene encoding a 610 amino acid nuclear protein, menin. Although mutations of the MEN1 gene are responsible for MEN 1 syndrome, the intracellular functions of menin have not been fully elucidated. Recent data suggest that interactions between menin and menin-interacting proteins have a role in physiological regulation of cell growth, control of the cell cycle and genome stability, and are potentially important in bone development and multipotent mesenchymal stem cell differentiation. Loss of these interactions might also contribute to the development of MEN 1 syndrome.