5-methylcytosine (5mC) in cytosine-guanine dinucleotide (CpG) is a usual epigenetic modification in mammals. It plays crucial roles in gene regulation, development, genomic imprinting and so on. In the last three years, it was discovered that in addition to 5mC, another modified cytosine base-5-hydroxymethylcytosine (5hmC) was abundant in many mammalian tissues, which may have different biological function from 5mC. This paper reviews the recent progresses in the studies of 5hmC.