HTRA4 and cancer: Using the AO/EB double staining and flow cytometry connected with annexin V and 7-aminoactinomycin D staining, we showed that exogenous production of the HtrA4/ΔN-HtrA4 in the lung adenocarcinoma A549 cells decreased the viability of etoposide-treated cancer cells by increasing the percentage of cells in the early and late stages of apoptosis (Figure 4 and Figure 5A, Table 2).