To determine whether the levels of TopBP1 in those TopBP1-depleted cells infected with high titers of Ad-TopBP1 were still within physiological ranges compared with the endogenous TopBP1 protein levels in other cancer cells and tissues, we compared TopBP1 expression among these cells and three commonly used cancer cell lines, including OVCAR3, C33A, and K562 cells (Fig. 1C, left panel). Here, TOPBP1 is linked to cancer.