We report here the alteration(s) in the expression of the DNA repair gene, DNA polymerase β, in kidney tumors induced by stilbene estrogen (diethylstilbestrol, DES). RT-PCR, slot blotting, and Northern blotting experiments revealed that expression of DNA polymerase β (DNA pol β) was several fold lower in stilbene-estrogen-induced kidney tumors than in age-matched controls. Several mutations were identified in DNA pol β mRNA from DES-induced kidney tumors, but not in age-matched control kidney. The mutations in DNA pol β mainly occurred in the catalytic domain of pol β, and not in the DNA binding domain. All the mutations produced a stop codon at nucleotide 199 indicating that a protein of aberrant size may be synthesized. These data suggest that mutation of DNA pol β coupled with attenuation in expression might comprise the DNA repair system. This in turn may allow a greater error rate during DNA repair and the accumulation of lesions in the genome.