The trimethoprim-PEG3-amine (TMP-PEG3-amine) can be conjugated to various probes to make trimethoprim-based chemical tag (TMP-tag). The TMP-tag is based on the high affinity (Kd ~ 1 nM) binding of TMP to the E.coli dihydrofolate reductase (eDHFR). The target protein, fused to eDHFR via standard molecular cloning technology, is labeled by the TMP-probe in a non-covalent manner. The low affinity of TMP with mammalian DHFR (Kd > 1 uM) ensures minimal background. The TMP-tag has been demonstrated to enable protein visualization and manipulation of both intracellular and surface targets in mammalian cells. The TMP-PEG-amine can be used in combination with our Mix-n-Stain™ Small Ligand Labeling Kits to generate fluorescent chemical tags without purification for cell surface or intracellular staining.