OBJECTIVES: Osterix (Osx) is an important transcription factor for bone formation which was identified through the study of Osx homozygous null mutants showing a complete absence of intramembranous and endochondral bone formation. However, due to the immediate death of Osx null mutants in the perinatal period, it is not possible to determine the role of Osx after birth. To study whether Osx is essential for bone formation and maintenance after birth, the function of Osx was examined in adult bones using the time- and site-specific Cre/loxP recombination system. In a previous study, Osx inactivation with 2.3-kb Col1a1-Cre exhibited the osteopenic phenotype in growing bones with an increase in early marker genes for osteoblast differentiation and a reduction of the late marker gene. Here, the difference of Osx downstream target genes was examined in Osx-inactivated mice with Col1a1-Cre. MATERIALS & METHODS: To further elucidate the difference in Osx downstream target genes in skeletal development, Osx-inactivated mice with 2.3-kb Col1a1-Cre were generated by crossing with conditional Osx knockout mice and 2.3-kb Col1a1-Cre. DNA microarray analysis was conducted in the long bones of these mice. RESULTS: Osx-inactivated mice were generated with a disorganization of collagen layer in bone phenotype, which was shown in a previous report. In Osx-inactivated bones, genes required for bone formation, such as Dlx, Smad, and Runx2, were increased, whereas cartilage-related genes were decreased. CONCLUSIONS: These results indicate that gene alterations by Osx expression will provide an understanding of the change in the in vivo microenvironment during postnatal bone formation and maintenance.