ABSTRACT. In adult mouse, the mRNA corresponding to the alpha1 chain of type XII collagen (alpha 1(XII)) is predominantly detected in the bone. Additionally, murine osteoblastic cells, MC3T3-E1, increased the mRNA level of alpha 1(XII) response to the mechanical strain in the stretch culture system. Cyclic stretch stress resulted in a threefold increase in mRNA level of alpha 1(XII) as compared to the control experiment in MC3T3-E1. Transient transfection assays employing a reporter construct, together with site-directed mutagenesis studies, suggested that the AP-1 binding site in the first exon of mouse alpha 1(XII) gene is important for stretch stressmediated upregulation of alpha 1(XII) expression. Electrophoretic mobility shift assay and associated antibody supershift experiments showed that stretch stress promotes the binding of c-Jun and JunD. Further chromatin immunoprecipitation experiments confirmed the participation of these transcription factors in the region. Also, the exogenous induction of the dominant negative form of c-Jun canceled the effect of stretch stress on the stimulation of the alpha 1(XII) gene. Here, we reported a potential responsive element to the stretch stress in mouse alpha 1(XII) gene. These data will provide new information on the mechanical strain-mediated transcriptional control of alpha 1(XII)-mediated fibrillogenesis in the bone.