Ionizing radiation and other oxidative stresses modify the bases in DNA. Several of these bases are substrates for DNA repair enzymes which is evidence that they are also modified by endogenous oxidants. This proposal continues to study formation of derivatives of thymine and cytosine in DNA exposed to oxidative stress. The potential mutagenicity of these modified bases will be studied. A repair enzyme, 5-hydroxymethyluracil-N- glycosylase will be purified so that its genetic expression may be studied. 32P-post labelling techniques for identification of these bases in vivo will be developed.