CypHer5E NHS ester is an excellent building block for developing site-specific pH probes. It can be readily conjugated to amino-containing biomolecules (such as antibodies, peptides and amino-modified oligos). CypHer5E is a red laser-excitable, pH-sensitive cyanine dye derivative that can be used to study most classes of cell surface receptors which internalize into the endocytic pathway when activated by a ligand. CypHer5E NHS ester is a flexible, generic fluorophore that can be coupled to a receptor or epitope tag antibody and monitors neutral to acidic intracellular events. CypHer5E has an excitation peak that perfectly matches the red laser at 633 nm of He-Ne laser and 647 nm of red laser and an emission peak at ~670 nm.