This construct was further extended for in vivo visualization and tracking of cells and the PDGFR protein. Biodistribution experiments show that GH680 was rapidly eliminated via renal excretion with lung, muscle, and liver fluorescence minimal within 4 hours after injection. Importantly there was no non-specific binding to muscle tissue where titin is expressed abundantly. To determine whether we could image cells in vivo using this system, SKI II IQ-tag expressing cells were first implanted subcutaneously into nude mice, followed by a systemic injection of GH680 and imaging by fluorescence mediated tomography. A number of reporter and imaging technologies have recently been developed to visualize site-specific proteins and cellular trafficking. Specific examples include protein tags, such as tetracysteine or hexahistidine motifs, which are recognized by biarsenic-derivitized fluorescein, his-tags for Ni-NTA-conjugated fluorochromes, or Dropropizine various forms of biotin ligases that are revealed with labeled avidins. Collectively, these methods have allowed unprecedented insight into cellular protein trafficking. Based on the hypothesis that it is feasible to develop affinity ligands to commercially available indolium fluorochromes that are commonly utilized in vivo, we performed de novo phage screens for high affinity binding peptides. Here we report on a novel peptide tag for NIR dyes, which is short, linear, and does not occur in nature. The identified IQ-tag peptide isunique and has a subnanomolar binding affinity for the NIR fluorochrome GH680, due to a number of optimal binding interactions between the peptide and benzindolium dye. According to results obtained from molecular modeling studies of the docked ligand, p-stacking between the phenylalanine residues of IQ-tag and the indolium subunit of the fluorophore yields preferential orientation of the peptide on the dye. The next three amino acids, SPH, extend over the alkene linker of the dye to the opposite face with the hydrophilic portions directed away from the core. Finally, the hydrophobic N-terminal isoleucine is able to interact with the similarly hydrophobic polymethine linker.