5-Hydroxymethylcytosine is a DNA pyrimidine nitrogen base and a nucleobase analogue of cytosine, where the hydrogen at position 5 is replaced by a hydroxymethyl group. Chemically, 5-hydroxymethylcytosine is an aminopyrimidine and an intermediate metabolite in active DNA demethylation. It plays a key role in gene expression, DNA methylation, and various critical cellular functions. 5-Hydroxymethylcytosine has been shown to be a potential biomarker of cellular transformation and is involved in axonal growth, transcriptional regulation, and the action of leukemia inhibitory factor. Application： 5-Hydroxymethylcytosine finds application in metabolomics, biochemical, phytochemical, cell signaling and drug discovery research.