We first investigated the expression of COX-2 protein in three breast cancer cell lines (MDA-MB-231, MDA-MB-435, and MCF-7) and an ovarian cancer cell line (SKOV3.ip1), all stably transfected with E1A, to determine the relationship between COX-2 protein expression level and sensitivity to celecoxib. The gene discussed is PTGS2; the disease is breast cancer.