S100 Ca2+-binding proteins became of major interest because of their differential expression in tissues and their association with human diseases. Earlier studies showed that 13 S100 genes are located as a cluster on human chromosome 1q21. Since a number of mouse S 100 genes, such as S100A4 and S100A6, have been localized to a syntenic region on mouse chromosome 3, we investigated if the S100 gene cluster exists in mouse and is structurally conserved during evolution. First we identified the cDNA sequences of mouse S100A1, S100A3 and S100A5. Then we isolated a 490 kb mouse YAC clone which gives a specific signal by FISH most likely on chromosome 3. Hybridization studies with different mouse S100 cDNAs revealed that eight mouse S100 genes are arranged in a clustered organization similar to that in human. The linkage relationships between the genes S100A8-S100A9 and S100A3-S100A4-S100A5-S100A6 were conserved during divergence of human and mouse about 70 million years ago. However, the separation of the mouse S100 genes S100A1 and S100A13 in comparison to the human linkage group suggests rearrangement processes between human and mouse. Our data demonstrate that the S100 gene cluster is structurally conserved during evolution. Further studies on the genomic organization of the S100 genes including various species could generate new insights into gene regulatory processes and phylogenetic relationships.