Discovered in 2011 in mammalian embryonic stem cells by Thomas Carell's research group[1] the modified nucleoside was more recently confirmed to be relevant both as an intermediate in the active demethylation pathway and as a standalone epigenetic marker.

[4] Similarly to the related cytosine modifications 5-Methylcytosine (5mC) and 5hmC, 5fC is broadly distributed across the mammalian genome, although it is much more rarely occurring.

Firstly, 5fC serves as an intermediate of the active demethylation pathway, a process that contributes to the DNA maintenance and integrity by replacing 5mC with canonical cytosine.

A central dilemma regarding 5fC (and epigenetics in general) is how reader proteins recognise their substrates with such high specificity over the overwhelming background.

Thymine-DNA glycosylase (TDG), a protein which is involved in the removal of 5fC from DNA in mammals, is especially interesting in this context.