We describe the sequence and expression pattern of Sp5, a novel member of the vertebrate Sp1 transcription factor gene family which consists of at least five members. This gene family is characterized by a highly conserved domain which is formed by three Zn fingers, which bind to the GC box or the GT/CACC box in the promoter of many genes. These boxes are important cis-acting elements required for the expression of the respective genes. In vitro experiments indicate that the Sp1 transcription factors act by influencing the methylation state of the DNA, or by direct interactions with other promoter specific transcription factors. Despite intensive research, the results from in vivo experiments, including targeted gene inactivation, have been difficult to explain. This may be due to possible redundancies and interferences with other transcription factors of this gene family. Here, we report the isolation of the mouse Sp5 gene, a novel Sp1 homolog. Its sequence indicates that Sp5 is a possible link between Sp1 and the closely related BTEB/KLF gene family. We provide detailed information of its highly dynamic expression pattern during mouse embryogenesis in the developing brain, the spinal cord, the trigeminal ganglia, the somites and additional sites outside the nervous system starting from embryonic day 7.25 (E7.25) up to E10.5.