1-(2-D-ribofuranosyl)-5-(carboxymethyl)uracil, also known as 5-carboxymethyluridine (cm⁵U), is a modified nucleoside. Its structure consists of a uracil base with a carboxymethyl group at the 5-position, which is glycosidically linked to a D-ribofuranose sugar moiety. This modification is a key feature distinguishing it from the canonical nucleoside uridine. This compound is primarily of interest in biochemistry and molecular biology research. It is a naturally occurring modified nucleoside found in the transfer RNA (tRNA) of various organisms, including bacteria and eukaryotes. Its presence in specific tRNA positions is believed to influence translational fidelity and efficiency. Consequently, cm⁵U and its derivatives are valuable as research tools for studying RNA modification, structure, and function, as well as for investigating related enzymatic pathways. As a modified nucleoside building block, it serves as a crucial intermediate in the chemical or enzymatic synthesis of oligonucleotides containing this specific modification. This enables the preparation of tailor-made RNA molecules for advanced biochemical and biophysical studies, contributing to the field of epitranscriptomics.