The marine demosponge Geodia cydonium encodes a gene, termed GCXPB, which displays 62% identity to the human XPB/ERCC-3 gene that specifically corrects the repair defect in xeroderma pigmentosum and in Cockayne's syndrome. The cDNA was isolated and characterized the deduced aa sequence, XPB_GEOCY, with the calculated size of 91,541 Da comprises the characteristic domains found in the related helicases. Phylogenetic tree analysis revealed that the sponge sequence is grouped to the metazoan related XPB/ERCC-3 polypeptides. Northern Blot analyses have been performed with sponge samples collected at different depths, thus exposed to different intensities of UV sunlight in the field. The intensity of the 2.6 kb band, corresponding to the transcripts of the sponge GCXPB gene was highest in those biotopes, which are closer to the surface of the sea, lower were the expressions in animals from a cave or from depths of 22 to 35 m. Controlled laboratory studies revealed that after irradiation of specimens with 300 or 1000 J/m2 UVB light a dose-dependent increase of the steady-state level of GCXPB occurs, values up to 29-fold with respect to the controls which were kept in the dark have been determined. In parallel, the DNA integrity in the sponge samples was measured using the sensitive Fast Micromethod assay. The data revealed that the degree of strand DNA breaks paralleled the increase of expression of the GCXPB gene. From these data it is concluded that the XPB/ERCC-3-like gene in the sponge G. cydonium is UV light-inducible and hence might be used as biomarker for UV light exposure in the field.