Figure 1:

Structures and inclusion chemistries of common RNA metabolic labels. A) 4-thiouracil variants and pathways for incorporation into nucleotide metabolism; once the nucleotide monophosphate is formed, the resulting compound is readily incorporated into cellular RNA. B) Structure of 5-bromouridine, which can be assimilated through the uridine kinase pathway as on the right side of panel A. C) Structure of the click chemistry substrate 5-ethynyluridine, again typically incorporated into the cellular nucleotide pool via uridine kinase activity.