MG (pyrimido[1,2-a]purin-10(3H)-one) is a heterocyclic compound which is a by-product of base excision repair (BER) of a specific type of DNA adduct called MdG. The MdG adduct in turn is formed by a condensation reaction between guanosine nucleotides in DNA and either malondialdehyde (propanedial)  or acrolein. If not repaired, these adducts are mutagenic and carcinogenic.
Malondialdehyde is an end product of lipid peroxidation while acrolein is a result of DNA peroxidation.
MdG is the major endogenous DNA adduct in humans. MdG adducts have been detected in cell DNA in liver, leucocytes, pancreas and breast in concentrations of 1-120 per 10 nucleotides. Detection and quantification of MdG adducts in the body as measured by free MG is a tool for detecting DNA damage that may lead to cancer. Free MG is also biomarker for oxidative stress.