Cell-penetrating peptides (CPPs), also referred to as peptide transduction domains (PTDs), are polypeptide domains that can enter many, if not most, cell types. CPP/PTDs mediate transduction into cells of a wide range of cargos that otherwise lack bioavailability, such as peptides, proteins, antisense oligonucleotides, and small interfering RNAs. CPP/PTDs are thus being studied extensively as delivery agents for molecular therapies, and a variety of transducing peptides have now been identified, including both naturally occurring domains and synthetically derived sequences comprising polycationic or amphipathic residues.1 Although much progress has been made in understanding the endocytotic mechanisms by which CPP/PTDs enter cells,1 several important unanswered questions remain. In an upcoming issue of Molecular Therapy, Hirose and colleagues report a study in which they reexamined the direct membrane transduction mechanism of CPP/PTDs using highly sensitive imaging techniques and well-controlled experimental systems.2 The study reinforces the notion that CPP/PTDs enter cells by endocytosis (macropinocytosis), but it also highlights the significance of the specific peptide and cargo conditions that drive membrane deformations required for concomitant, low-level nonendocytotic uptake.