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Runx2 (runt-related transcription factor 2, also known as Cbfa1, Osf2 and AML3) is essential for bone development in mice, and mutations in RUNX2 are found in 65–80% of individuals with cleidocranial dysplasia 1, 2. Although all Runx family members can interact with Cbfβ (core-binding factor b, encoded by Cbfb), a role for Cbfβ in bone development has not been demonstrated owing to lethality in Cbfb−/− mouse embryos at 12.5 days post coitum (dpc) from hemorrhages and lack of definitive hematopoiesis 3, 4. Using a'knock-in'strategy, we generated mouse embryonic stem (ES) cells that express Cbfb fused in-frame to a cDNA encoding green fluorescent protein (GFP) 5. Cbfb+/GFP mice had normal life spans and appeared normal, but Cbfb GFP/GFP pups died within the first day after birth. The Cbfb GFP/GFP mice exhibited a delay in endochondral and intramembranous ossification as well as in chondrocyte …