The C7C library features a random fragment flanked by a pair of cysteine residues. These cysteines are oxidized to form a disulfide bond during phage assembly, resulting in the displayed peptide being presented to the target in a cyclic conformation. Each library possesses a complexity of 10⁹ independent clones, sufficient to encode most possible 7-mer (1.28 × 10⁹) peptide sequences. However, this represents only a small fraction (less than one millionth) of the approximately 4.1 × 10¹⁵ possible 12-mer sequences. Therefore, the 12-mer library is generally considered to have diversity equivalent to the 7-mer library, but distributed across 12 residues.

In both 7-mer and 12-mer libraries, the N-terminal first residue of the peptide-pIII fusion is the first randomized position. In the C7C library, the randomized positions are preceded at the N-terminus by Ala-Cys. All libraries incorporate a short linker sequence (Gly-Gly-Gly-Ser) between the displayed peptide and the pIII coat protein.

The primary strategy for Phage Peptide Libraries Screening leverages the varying affinities between biomolecules. Using a biomolecule as the target, specific binding peptides with defined affinity can be isolated from the peptide library through direct or multiple rounds of screening. Phage Peptide Library Screening typically employs biological screening techniques, also known as Biopanning, which use biological operations to isolate desired phage peptides from a random phage peptide library. Screening methods can vary based on the Nature of the Target, the Scale of Screening, and Operator Preference. 

The main steps of Library Screening are:

1. Select Phage Target Capture Method: Prepare the capture surface using target coating and blocking, or choose the affinity magnetic bead method.

2. Incubate the Phage Library with the Target.

3. Wash away unbound Phage using TBST, and elute Bound Phage using low pH buffer or known ligands to the target.

4. Amplify the selected Phage eluate. Concentrate phage by adding PEG/2.5 M NaCl to the culture supernatant.

5. Titer the unamplified eluate and the enriched phage pool to determine the input for the next round of Selection. Increase selection or wash stringency as needed.

6. After Three Rounds of Screening, do not amplify the eluate. Instead, titer the third-round eluate to obtain individual plaques for Phage Sequencing Reactions.

7. Post-Screening Analysis: Determine sequence motifs and/or conduct binding studies with selected clones.

Two drugs developed through random peptide libraries of Phage Display Screening are peginesatide (marketed as Hematide by Affymax) and romiplostim (marketed as Nplate by Amgen). Peginesatide is a pegylated erythropoietin receptor agonist, while romiplostim is an Fc-fused thrombopoietin receptor agonist. Using Phage Display Technology, two other drugs have also been developed: the anti-TNF-α antibody adalimumab and the small protein inhibitor of plasma protease kallikrein, ecallantide. This success establishes the technique alongside other mature methods in the Drug Discovery field.