Phage display is a method to discover peptide ligands while minimizing and optimizing the structure and function of proteins (Hallahan, 2003). The phage is used as a scaffold to display recombinant libraries of peptides and provides a means to recover and amplify the peptides that bind to putative receptor molecules in vivo. In vivo selection simultaneously provides positive and subtractive screens because organs and tissues such as tumors are spatially separated. Phage DNA can then be sequenced to determine the amino acid sequence of peptides on the capsid that have been recovered from tumors. The T7 phage display system exploits the T7 capsid protein as a scaffold to display peptides on the capsid protein unique to the 10B protein on the surface of the phage. Gene 10 encoding the capsid protein is cloned with a series of multiple cloning sites at the C-terminus of the 10B protein. The natural translational frame shift site within the capsid gene has been removed so that only a single form of the capsid protein is made. This results in a total of 415 peptides displayed on the surface of the phage.