tokens: To investigate the role of bHLH proteins in MC3T3 - E1 osteoblasts , which undergo a developmental sequence in vitro , we analyzed the transcriptional control of osteocalcin gene expression by stable transfection of an osteocalcin promoter - luciferase chimeric gene ( p637OC - luc ) and assessed the role of E - box cis - acting elements in osteocalcin promoter by DNA binding assays .