This peptide consists of two functional domains, on the one hand a "homing" or targeting domain, which guides the peptide to targeted cells and allows its internalization and on the other hand a programmed cell death-inducing sequence. The targeting domain contains the Asn-Gly-Arg (NGR) motif which has proven useful for delivering various anti-tumor compounds and viral particles to tumor vessels. As programmed cell death-inducing sequence the synthetic 14 amino acid peptide klaklakklaklak, also called (klaklak)2, was selected, because it kills bacteria at concentrations 1% of those required to kill eukaryotic cells. This pro-apoptotic domain is non-toxic outside of cells, but toxic when internalized into targeted cells by the disruption of mitochondrial membranes. Therefore, targeted pro-apoptotic peptides represent a potential new class of anti-cancer agents. They combine two levels of specificity: "homing" to targeted cells and selective apoptosis of such cells after entry.