4-Thiouridine (CAS 13957-31-8) is a thiol-substituted ribonucleoside analog commonly utilized in RNA research for photoactivatable labeling and RNA cross-linking experiments Structurally 4-thiouridine differs from natural uridine solely by the substitution of a sulfur atom at the 4-position of the nucleotide base Due to this close structural similarity it integrates into RNA sequences with minimal disruption preserving typical base-pairing properties and RNA functionality Upon UV irradiation the incorporated thiol group induces covalent short-range RNA RNA cross-links facilitating detailed analysis of RNA interactions turnover rates and dynamics within cellular contexts