RUNX2 and osteosarcoma: U2OS osteosarcoma cells are a good model for osteoblasts,20, 21 and GATA4 biology can be largely reproduced in this cell line.6 Indeed, GATA4 can be localized by ChIP to the Runx2 promoters and enhancer region, as in primary calvarial osteoblasts, and silencing of Gata4 reduces Runx2 expression (Supplemental Fig. S7A).