Bone morphogenetic proteins (BMPs) are characterized by their ability to induce osteoblastic differentiation. However, the mechanism of osteo-induction by BMPs has yet to be determined. Using differential display we previously identified AJ18, a zinc finger transcription factor, as an immediate-early response gene to BMP-7. AJ18 was shown to bind to the osteoblast-specific element2 (OSE2) and to modulate transactivation by Runx2, a master gene in osteoblastic differentiation. Here we describe the temporal and spatial expression of AJ18 in developing mouse tissues. AJ18 mRNA expression was observed in most tissues, except liver, and was generally highest early in embryonic development, decreasing markedly after parturition. Consistent with immunohistochemical analysis, AJ18 mRNA expression was highest in the brain, kidney, and bone of 17 dpc (days post coitum) embryos. In endochondral bones of embryonic and 4-week-old mice, immunostaining for AJ18 was strong in the nuclei of proliferating and pre-hypertrophic chondrocytes, and osteoblasts, whereas there was low or no staining in hypertrophic chondrocytes. In teeth of embryonic and 4-week-old mice, nuclear staining was observed in precursor and mature ameloblasts, odontoblasts, and cementoblasts, respectively. In addition, in 4-week-old mice staining of AJ18 was observed within alveolar bone cells and periodontal ligament cells. In general, the spatial expression of AJ18 in skeletal and non-skeletal tissues of mouse embryos showed striking similarity to the expression of BMP-7 mRNA. Therefore, the expression of AJ18 is consistent with its perceived role as a transcriptional factor that regulates developmental processes downstream of BMP-7.