Lightcycler® 640 (NHS Ester) is only available as an NHS Ester and must be attached to an amino-modified oligonucleotide. It is a red dye which spectrally lies between Texas Red® (NHS Ester) and Cy5™. It is commonly used as a FRET reporter along with a 6-FAM™ donor in assays run on the Roche Lightcycler® real-time PCR platform.