C10 is a recently described member of the beta-chemokine subfamily of the chemokine superfamily of cytokine proteins. Genomic clones encoding murine C10 were isolated and sequenced. The other members of the beta-chemokine family have a three-exon genomic structure containing, among other sequence similarities, four cysteines spaced in a highly conserved manner. In each of these genes, the second exon contains the first three of the four conserved cysteines, and the third exon contains the last. In contrast to this genomic structure, the C10 gene has four exons, with a novel second exon of 48 nucleotides. Exons 3 and 4 of C10 contain four cysteines distributed in the same manner as in exons 2 and 3 of other beta-chemokine family members. The novel second exon codes for a large number of charged amino acids, and this exon shows no homology to any previously described sequences in computer databases. Linkage studies showed that the C10 gene (Scya6) is closely linked to the Scya2 locus on mouse chromosome 11, indicating that the C10 gene is located in the same region of mouse chromosome 11 as other members of the beta-chemokine family. Thus, although the C10 gene contains a novel exon not found in any other members of the chemokine superfamily, its chromosomal location and conservation of cysteine residues and other structural features suggest that it evolved from the same ancestral gene as other members of the beta-chemokine family.