The present invention provides methods for inducing differentiation of undifferentiated mammalian cells into osteoblasts, and methods for identifying a compound that induces differentiation of undifferentiated mammalian cells into osteoblasts. In addition, the present invention provides polynucleotides and vectors, and the use thereof as a medicament for the treatment of a disease involving a systemic or local decrease in mean bone density. Moreover, the present invention provides methods for in vitro production of bone tissue, osteoblast cells and methods for diagnosing a pathological condition involving a systemic or local decrease in mean bone density or a susceptibility to the condition.