5-(Hydroxymethyl)cytidine (hm⁵C) is a modified nucleoside where a hydroxymethyl group is attached to the 5-position of the cytosine base. It is a naturally occurring RNA modification, specifically found in transfer RNA (tRNA) and messenger RNA (mRNA) of various organisms, including mammals. Its primary significance lies in the field of epigenetics and RNA biology, where it is recognized as one of the key chemical modifications constituting the "epitranscriptome." hm⁵C plays a crucial role in regulating gene expression, RNA stability, and cellular differentiation. As a research tool, it is used as a standard or reference compound in analytical methods such as liquid chromatography-mass spectrometry (LC-MS) for the detection and quantification of RNA modifications. Due to its involvement in fundamental biological processes and potential links to development and disease, hm⁵C is an important subject of study in life science research, particularly in investigations concerning RNA modification mapping, dynamics, and functional consequences.