Utilizing in vivo phage selection, phage display has been used to identify and isolate peptides capable of mediating selective localization to various organs such as brain and kidney (Pasqualini and Ruoslohti (1996) Nature 380:364-366) as well as to identify peptides that home to particular tumor types bearing ??v??3 or ??v??5 integrins (Arap et al., (1998) Science 279:377-380).