Reversible terminators can be classified into two main groups based on the difference of reversible blocking groups.

• 3’-O-blocked reversible terminators: The blocking group is linked to the oxygen atom of the 3’-OH of pentose, while the fluorescence tag is linked to the base. Fluorescent tag acts as a reporter and can be cleaved.
• 3’-unblocked reversible terminators: The reversible termination group is linked to the base as well as the fluorescence group, which not only is a reporter but also functions as part of the reversible terminating group. Since the 3’-OH is unmodified, this type of terminator is easier to be accepted by the DNA polymerases. However, the termination effect is not as good as the 3’-O-blocked ones.