activation 1 the action or process of rendering an atom, molecule, or other substance reactive or more reactive, whether physicochemically, chemically, or biochemically. 2 the process of rendering material artificially radioactive: radioactivation. See radioactivation analysis. 3 the initial changes in an ovum during fertilization, covering the periodfrom first contact with a sperm to dissolution of

acyclovir or acycloguanosine 9-(2-hydroxyethoxymethyl)guanine; an antiviral agent widely used in the treatment of human herpes infections. It is selectively phosphorylated by herpesvirus-induced thymidine kinase and the phosphorylated compound is a potent inhibitor of herpesvirus-induced DNA polymerase. One proprietary name is Zovirax.

activin one of two gonadal glycoproteins related to transforming growth factor-b (the other is inhibin); present in two forms in human gonads, it exists as a dimer of inhibin bA or bB chains, linked by disulfide bonds. Activin A is a dimer of bA chains; activin AB is a dimer of bA and bB chains.