The multiple endocrine neoplasia type 1 gene product, menin, interacts with Jun D. The physiological role of menin in cell cycle control and the manner in which its inactivation contributes to tumorigenesis remain unknown. In the present study, the expression of menin was examined at various cell cycle stages in GH4C1 cells, a rat pituitary cell line. Cells synchronized at the G1-S-phase boundary expressed menin at a lower level than G0-G1-synchronized cells. The expression of menin increased as the cells entered S phase, at which time Jun D expression also increased. In contrast, cells synchronized at the G2-M phase expressed lower levels of menin. At G0-G1, G1-S, and G2-M phases of the cell cycle, menin was found predominantly in the nucleus. In summary, we show that in pituitary cells, menin is a nuclear protein whose expression is cell-cycle regulated. The data suggest that menin has an important role in cell growth regulation.