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Short answer: Cell-penetrating peptides (CPPs) are short peptides, usually 5 to 30 amino acids, that cross cell membranes and can carry cargo with them. The best-known examples are TAT (from HIV-1), penetratin (from the Antennapedia homeodomain), transportan (a chimera of wasp-venom and galanin fragments) and synthetic polyarginine such as R8. Others include VP22, SynB1, LMWP, CADY-1, iRGD and LyP-1.
Definition and scope
The 2021 review in Pharmaceutics describes CPPs, formerly known as protein transduction domains (PTDs), as a class of 5 to 30 residue peptides able to pass through biological membranes in an energy-dependent or energy-independent way. A 2019 review in Biomolecules gives a slightly different range, 6 to 30 amino acids, and notes that CPPs generally have a high content of basic amino acids and so an overall positive net charge. The two ranges differ because the field has no single formal definition.
Scale matters when you search for examples. CPPsite 2.0, a database maintained by IIIT-Delhi, lists around 1,700 unique experimentally validated CPPs, linear or cyclic, some containing modified or non-natural residues, with cargo types that include nucleic acids, proteins, nanoparticles and peptide nucleic acids (PNA). The handful of named peptides below are a small, frequently cited fraction of that set.
How CPPs enter cells
The 2019 review separates uptake into energy-independent direct penetration and energy-dependent endocytosis. Direct models include transient pores (barrel-stave and toroidal), the carpet model and inverted micelles. Endocytic routes include macropinocytosis, clathrin-mediated, caveolae or lipid-raft-mediated, and clathrin- and caveolae-independent endocytosis. The 2019 reviewers state that the exact mechanism of CPP cell penetration remains unknown, and that the mechanism of endosomal escape is still poorly understood.
The 2021 review lists the cargo types attached to CPPs in cancer research over the last decade: siRNA, plasmid DNA, mRNA, Cas9 and sgRNA, proteins and peptides, and chemotherapeutic drugs.
Cell-penetrating peptides examples: sequences and origins
The table collects examples from the two reviews listed under Sources. Sequences are given as printed there; a dash means no sequence is printed. Check the cited papers before using any sequence in experimental work.
| Peptide | Origin or class as described | Sequence as printed in the reviews |
|---|---|---|
| TAT | Derived from the HIV-1 Trans-Activator of Transcription protein; natural protein-derived | GRKKRRQRRRP |
| Penetratin | 16-residue peptide from the third helix of the Antennapedia homeodomain; natural protein-derived | – |
| Transportan | Chimeric: 14 amino acids from mastoparan and 12 from the N-terminal part of the neuropeptide galanin | – |
| R8 (oligoarginine) | Fully synthetic oligoarginine | RRRRRRRR |
| VP22 | Herpes simplex virus 1 (HSV-1) protein VP22 | DAATATRGRSAASRPTQRPRAPARSASRPRRPVE |
| SynB1 | Listed as a CPP in the 2021 review | RGGRLSYSRRRFSTSTGR |
| LMWP | Low molecular weight protamine | VSRRRRRRGGRRRR |
| CADY-1 | Listed as a CPP in the 2021 review | GLWWKAWWKAWWKSLWWRKRKRKA |
| iRGD | Listed as a CPP in the 2021 review | CRGDKGPDC |
| LyP-1 | Listed as a CPP in the 2021 review | CGNKRTRGC |
| Angiopep-2 | Listed among the CPP examples in the 2019 review | TFFYGGSRGKRNNFKTEEY |
Types: how CPPs are classified
The 2021 review groups CPPs in two ways (by charge and by source), and the same peptide can be placed on both axes.
By charge and structure
Cationic CPPs are rich in basic residues, histidine, lysine and especially arginine. Amphipathic CPPs combine charged and hydrophobic character. Linear versus cyclic form is a third property: CPPsite records both linear and cyclic peptides.
By source
Natural protein-derived CPPs include penetratin and TAT. Chimeric CPPs such as transportan join fragments of different proteins: 14 amino acids from mastoparan and 12 from the N-terminal part of galanin. Fully synthetic CPPs include oligoarginines (R8 in the table) and peptide nucleic acids (PNAs).
A three-question checklist for reading any CPP name
- Where did it come from? Natural protein fragment, chimera or fully synthetic.
- What is its charge character? Cationic (arginine, lysine, histidine rich) or amphipathic.
- Is it linear or cyclic, and does it carry modified residues? CPPsite records both linear and cyclic peptides, including non-natural residues.
Where it matters for labs and clinics
The 2021 review names three recurring limitations of CPPs: susceptibility to proteolytic degradation, uptake into intracellular endosomes, and low cell specificity. It also states that no CPP-based drug had yet received US FDA approval for clinical application, although one peptide, p28, was in two phase I clinical trials; that statement is the review authors’ as of its 2021 publication; for current regulatory status consult FDA records directly.
For lab staff the practical consequence is that a CPP is a research reagent class with open questions, not an established clinical product. Methods used to get nucleic acids into cultured cells are covered in the CASRAI guides on transfection methods and biosafety approval, electroporation parameters and the lentiviral transduction protocol. The electroporator guide explains the instrument, and cell culture basics covers the culture work these reagents are used in.
Common misunderstandings
- Not all CPPs are cationic. Cationic and amphipathic classes both appear in the 2021 classification.
- There is no single uptake mechanism. Direct penetration and several endocytic routes are all described, and the 2019 review states the exact mechanism remains unknown.
- Entering the cell is not the same as reaching the target. Endosomal entrapment is one of the limitations the reviews list.
- A CPP is not a cargo. The cargo, such as siRNA, protein or nanoparticle, is attached to or mixed with the peptide.
FAQ
What are examples of cell-penetrating peptides?
TAT, penetratin, transportan, polyarginine such as R8, VP22, SynB1, LMWP, CADY-1, iRGD, LyP-1 and Angiopep-2 are examples listed in the two reviews cited below.
How long is a cell-penetrating peptide?
The 2021 review gives 5 to 30 residues; the 2019 review gives 6 to 30.
How are cell-penetrating peptides classified?
By charge, as cationic or amphipathic, and by source, as natural protein-derived, chimeric or fully synthetic.
Is any cell-penetrating peptide drug FDA approved?
The 2021 review states that no CPP-based drug had yet obtained US FDA approval at the time of its publication.
Sources
- Combination of Cell-Penetrating Peptides with Nanoparticles for Therapeutic Application: A Review. Biomolecules 2019. https://pmc.ncbi.nlm.nih.gov/articles/PMC6359287/ (checked 2026-10-29).
- Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade. Pharmaceutics 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8470549/ (checked 2026-10-29).
- CPPsite 2.0, IIIT-Delhi. https://webs.iiitd.edu.in/raghava/cppsite/ (checked 2026-10-29).
