Cell-penetrating peptides (CPPs), also known as peptide transduction domains (PTDs), are short peptides (< 40 amino acids) that efficiently pass through cell membranes while being covalently or non-covalently bound to various molecules, thus facilitating these molecules' entry into cells. Cell entry occurs primarily by endocytosis but other entry mechanisms also exist. Examples of cargo molecules of CPPs include nucleic acids, liposomes, and drugs of low molecular weight.
CPP cargo can be directed into specific cell organelles by incorporating localization sequences into CPP sequences. For example, nuclear localization sequences are commonly used to guide CPP cargo into the nucleus. For guidance into mitochondria, a mitochondrial targeting sequence can be used; this method is used in protofection (a technique that allows for foreign mitochondrial DNA to be inserted into cells' mitochondria).