ABCG2 and oral cavity squamous cell carcinoma: Expression of ABCG2 and Bmi-1 was rigorously evaluated in a collection of eight oral cell lines originating from normal tissue (OKF6-TERT2), mild dysplasia (DOK), severe dysplasia (POE-9n), and oral squamous cell carcinoma (PE/CA PJ15, SCC04, SCC25, SCC09, and SCC15).