dlx5 drives runx2 expression and osteogenic differentiation in developing cranial suture mesenchyme craniofacial bones derive from cephalic neural crest by endochondral or intramembranous ossification here we address the role of the homeobox transcription factor dlx5 during the initial steps of calvaria membranous differentiation and we show that dlx5 elicits runx2 induction and full osteoblast differentiation in embryonic suture mesenchyme grown in vitro first we compare dlx5 expression to bonerelated gene expression in the developing skull and mandibular bones we classify genes into three groups related to consecutive steps of ossification secondly we study dlx5 activity in osteoblast precursors by transfecting dlx5 into skull mesenchyme dissected prior to the onset of either dlx5 and runx2 expression or osteogenesis we find that dlx5 does not modify the proliferation rate or the expression of suture markers in the immature calvaria cells rather dlx5 initiates a complete osteogenic differentiation in these early primary cells by triggering runx2 osteopontin alkaline phosphatase and other gene expression according to the sequential temporal sequence observed during skull osteogenesis in vivo thirdly we show that bmp signaling activates dlx5 runx2 and alkaline phosphatase in those primary cultures and that a dominantnegative dlx factor interferes with the ability of the bmp pathway to activate runx2 expression together these data suggest a pivotal role of dlx5 and related dlx factors in the onset of differentiation of chick calvaria osteoblasts