TFRC and neoplasm: Martin et al. [108] demonstrated ligand-dependent pDNA delivery by designing cyclic RGD-PEG-Stp 2-arm oligoaminoamides (Figure 3E); the same strategy was successfully developed for the targeting peptide B6, which was initially assumed to enhance uptake via the transferrin receptor (TfR) but later on was discovered as an TfR independent tumor cell uptake facilitator [155,156].