Glucuronic acid (α-D-GLUCURONIC ACID) is a carboxylic acid that has the structure of a glucose molecule that has had its sixth carbon atom (of six total) oxidized. The salts of glucuronic acid are known as glucuronates. Glucuronic acid is highly soluble in water. In humans, glucuronic acid is often linked to toxic or poisonous substances to allow for subsequent elimination, and to hormones to allow for easier transport. These linkages involve O-glycosidic bonds. The process is known as glucuronidation, and the resulting substances are known as glucuronides (or glucuronosides). Glucuronidation uses UDP-glucuronic acid (glucuronic acid linked via a glycosidic bond to uridine diphosphate) as an intermediate. UDP-glucuronic acid is formed in the liver of all animals. α-D-GLUCURONIC ACID is used as pharmaceutical intermediate and in chemical research.

In mammalian as well as in plant DNA, and in the DNA of many other organisms, there occurs a fifth nucleotide, 5-methyldeoxycytidine (5-mC), in addition to the traditionally recognized four nucleotides A, C, G, and T. The modification of cytidine to 5-mC, apparently the only one among the nucleotides in mammalian DNA, is introduced postreplicationally by several DNA methyltransferases (DNMT) which are chosen depending on the functional context of their enzymatic activity: DNA can be methylated de novo, still a most enigmatic series of events, or a given pattern of DNA methylation in the genome can be maintained upon replication. In general, 5-mC can be considered as a modulator of protein-DNA interactions and genome activity.