PGE₂/EP4 signal targets a certain population of osteolineage cells for OPN induction. Assessment of OPN protein expression by flow cytometry in primary osteoblastic cell cultures from EP4^+/+ (A) and EP4^−/− (B) mice with or without PGE₂ treatment. The fluorescence intensity of each experiment is represented as the geometric mean (GeoMean) value by PE-anti-OPN (open red histogram) after subtraction of that by PE-mouse IgG (closed purple histogram; n = 3). Data are represented as mean ± SEM. *P < .05; ***P < .001.