Rhodamine Red-X modification is a post synthesis conjugation to a primary amino group. The amino group can be placed at the 5' and 3' and for internal positions an amino modified base is used, e.g Amino dT C6

Rhodamine Red X is a red fluorescent dye used for labeling oligonucleotides excitable by the 568 nm spectral line of the krypton/argon ion laser. Rhodamine Red X has an absorbance maximum of 570 nm and an emission maximum of 590 nm. Rhodamine Red X can be used to label oligos slated for typical molecular biology applications (RT-PCR, hybridization probes, etc). Rhodamine Red X is particularly useful in experiments requiring triple labeling along with Cy2 and Cy5 because Rhodamine Red X’s emission maximum lies midway between those of Cy2 and Cy5 and its spectrum has little overlap with these other fluorophores.