The gadd45 gene is transcriptionally activated through at least two different mechanisms; one following treatment with base-damaging agents such as methylmethane sulfonate and UV radiation and the other following ionizing radiation. To investigate the sequences involved in induction of gadd45 by agents producing high levels of base damage, the hamster, human, and mouse genes were sequenced. Comparison of these sequences revealed a high level of conservation between species of 1500 base pairs of the proximal promoter and 700 base pairs within the third intron. However, in the promoter regions, there was no conservation between species of any transcription factor binding sites known to confer DNA damage responsiveness. The promoter of the hamster gene was inducible by base-damaging agents in both rodent and human cell lines and the human gene was inducible in a rodent cell line. This indicates that both sequence elements in the gadd45 promoter and factors binding to these sites are conserved in mammalian cells. Deletion analysis of the hamster promoter did not reveal any specific sequence which conferred damage inducibility and the maximal response required a large portion of the promoter. The hamster promoter was not inducible by ionizing radiation, suggesting that sequences outside the promoter region used, such as a p53 binding site in the third intron, are necessary. The human GADD45 gene was mapped to chromosome 1p31.1-31.2.