Overview of activatable cell-penetrating peptides (ACPPs) based on side chain modifications. Amino acids are indicated via the single letter code, in which d-amino acids are noted in lower case. FITC = fluorescein; Luc PNA = luciferase peptide nucleic acid; Dox = doxorubicin; pen = penetratin; VB = vinorelbine bitartrate.

		Number of R groups	CPP	Cargo	Trigger	Ref.
Enzyme sensitive side chain modifications
1		1 or 2	Tat	FITC	Aminopeptidase N dipeptidyl peptidase IV	55
2		2	M918	Luc PNA	Bacterial azoreductase	57
pH sensitive side chain modifications
3		3	Tat	Dox	pH 5.0	58
4		6	CR8G3PK6	Dox	pH 6.8	59
Light sensitive side chain modifications
5		3	Pen	(klaklak)2	λ = 365 nm, 6 W, 10 min	60
6		3	Pen	VB	λ = 740 nm, 30 min, 3.48 × 1012 photons s−1	61