Dong et al. used a conjugate of polyethyleneimine (PEI) with DOX via a pH-responsive hydrazone linkage (PEI–Hz–DOX, PHD) and a tumor-targeting folate ligand conjugated to PEI using polyethylene glycol (PEG) as a linker (PEI–PEG–Folate, PPF) in tandem with siRNA targeting vascular endothelial growth factor (VEGF) [30]. The gene discussed is PDC; the disease is neoplasm.