Cyclic peptides are of considerable interest as potential protein ligands. Cell-permeable cyclic peptides receive much attention as peptide-based drugs owing to its cell permeability, specific targeting, and excellent stability in vivo. Nowadays, the researchers working on cell-permeable cyclic peptides focus on the issues as follow: new cell-permeable cyclic peptides from natural products and their structure-activity relationships, and the chemical modifications of cyclic or linear peptides to artificially obtain cell-permeable cyclic peptides. Here, we review the issues mentioned above. This article is arranged in two sections. The first section introduces two kinds of discovered natural products as cell-permeable cyclic peptides; the second section covers the strategies of modifying cyclic or linear peptides into cell-permeable cyclic peptides and the insight of the structure-activity relationship of cell-permeable cyclic peptides.

Contents
1 Introduction
2 Cell-permeable cyclic peptides from natural products
2.1 Cyclosporine A
2.2 Cyclotides
3 Cell-permeable cyclic peptides from artificial modification
3.1 Strategies of modification based on CSA
3.2 Strategies of modification based on cyclotides
3.3 Strategies of modification based on linear peptides
4 Conclusion