From Eterna Wiki

A single Cytosine Nucleotide floating in phase space on the eteRNA console
The chemical structure of cytosine.

Cytosine, abbreviated C, is a pyrimidine base. It appears green in EteRNA. Cytosine pairs with guanine. Under some conditions, it is suspected that Cytosine may spontaneously mutate to Uracil, possibly disrupting previously held bonds.