Notes: Cytosine-13C,15N2 is intended for use as an internal standard for the quantification of cytosine (Item No. 36219) by GC- or LC-MS. Cytosine is a pyrimidine base.{48688} It forms complementary base pairs with the purine guanine (Item No. 34248) in DNA and RNA and is a precursor of uracil (Item No. 26088). Methylation of cytosine is a key epigenetic mark that plays a role in gene regulation.{29364}