Cytosine (/ˈsaɪtəˌsiːn, -ˌziːn, -ˌsɪn/;[2][3] C) is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto group at position 2).