EREG and head and neck squamous cell carcinoma: The IC50 of erlotinib in 6 HNSCC cell lines is shown in Supplementary Figure S6A. Notably, the growth inhibition due to erlotinib treatment of HN4, HN30 and FaDu cells that expressed high EREG protein levels was significantly higher than that of cells expressing lower levels of EREG (Figure 9B-C).