The human genome contains large regions that are highly structured. Sequence-related members of multigene families are often found in a clustered organization. Here we describe a previously unrecognized gene cluster composed of genes coding for calcium-binding proteins of the S100 family. The linkage of six S100 genes was established by pulsed-field gel electrophoresis, and a contiguous DNA sequence of 15 kilobases containing the full coding region of four different S100 genes was characterized. This is the tightest mammalian gene cluster discovered so far to our knowledge. Two additional S100 genes are located within the cluster, both of which exhibit unique structural features when compared with other S100 genes. The product of S100E is cysteine-rich, whereas that of S100D contains a long hydrophobic N-terminal tail. The gene cluster was assigned to chromosome 1q21, one of the bands showing rearrangements in neoplasms at high frequency. The deregulated expression of some S100 genes in the cluster during tumor progression suggests that chromosomal abnormalities may influence the expression of S100 genes in late stages of cancer, particularly in association with the formation of metastases.