4-Thiouridine (4-SU) is a photoactivatable ribonucleoside analog that is widely used for RNA analysis, including short-range RNA-RNA crosslinking and nascent RNA labeling. The crosslinking thio moiety is attached directly to the nucleotide base, thus 4-SU differs from uridine only by a single sulfur substitution. This offers the advantage of incorporating into an RNA chain with minimal structural perturbation and with similar base-pairing properties, reducing the likelihood that substitution will impair RNA interactions or activities.