5-Methylcytosine is a methylated form of the DNA base cytosine (C) that regulates gene transcription and takes several other biological roles.

In 5-methylcytosine, a methyl group is attached to the 5th atom in the 6-atom ring, counting counterclockwise from the NH-bonded nitrogen at the six o'clock position.

[2] The nucleic acid was found to be unusual, in that it contained in addition to thymine, guanine and cytosine, a methylated nucleotide.

[3][4] This report was severely criticized because their identification was based solely on the optical properties of the crystalline picrate, and other scientists failed to reproduce the same result.

[14] DNMT1 contains the replication foci targeting sequence (RFTS) and the CXXC domain which catalyze the addition of 5mC marks.

[16] The mechanism of the addition is as follows: first a cysteine residue on the DNMT's PCQ motif creates a nucleophillic attack at carbon 6 on the cytosine nucleotide that is to be methylated.

Passive DNA demethylation by dilution eliminates the mark gradually through replication by a lack of maintenance by DNMT.

[1] TDG knockout produced a 2-fold increase of 5fC without any statistically significant change to levels of 5hmC, indicating 5mC must be iteratively oxidized at least twice before its full demethylation.

[16] Repeat sequences in the genome, including satellite DNA, Alu, and long interspersed elements (LINE), are often seen hypomethylated in cancer, resulting in expression of these normally silenced genes, and levels are often significant markers of tumor progression.

[27] Hannum's clock utilizes adult blood samples to calculate age based on an orthogonal basis of 71 CpGs.

[28] Levine's clock, known as DNAm PhenoAge, depends on 513 CpGs and surpasses the other age estimators in predicting mortality and lifespan, yet displays bias with non-blood tissues.