Mouse neuroD/β2 promoter activity (−419/+59 bp) was examined in the presence of the CMV-driven expression vector containing either INSM1, cyclin D1 or both. Two different cell lines, D283Med (open box) and HEK-293 (shaded box), were used as either INSM1-expressing (D283Med) or non-expressing (HEK-293) cells. INSM1 alone induces a 40–50% inhibition of neuroD/β2 promoter activity in both cell lines, whereas cyclin D1 alone induces a 50–60% inhibition of the neuroD/β2 promoter activity in D283Med, but not in HEK-293, cells. The combined expression of INSM1 and cyclin D1 enhances inhibition of neuroD/β2 promoter activity by up to 70% (D283Med, *P<0.01). A CMV-β-galactosidase construct was used to normalize the transfection efficiency. The graph shows the means for four separate experiments±S.E.M. A representative Western blot of the expressed proteins is shown below the histogram.