4-Thiouridine
4-Thiouridine is an atypical nucleoside formed with the 4-thiouracil base found in transfer RNA. Its biosynthesis has been determined.
The IUPAC reserved the single-letter nucleobase code "S" for thiouridine in 1970, but the convention does not appear very common. "S" was repurposed to mean "C or G" in 1984.