tertiary structures. These C chemokines also have the same flanking regions, meaning regions of the gene including the promoter and other places of protein binging that do not contribute to the RNA transcribed gene. Gene mapping of this chemokine family shows similarities in their intron and exon locations with only one distinct difference. XCL1 has only one difference in its first intron that encodes for a large ribosomal subunit called L7a. In XCL2 have of the region encoding for L7a is cut off. The only other genetic difference between the two mature proteins is the different amino acid in positions 7 and 8. This amino acid difference may account for some biological differences. Some difficulties with comparing these two chemokines is that XCL2 has never been observed in a mouse. Structure One thing that sets XCL1 apart from other cytokines is its structure. While most chemokines have two disulfide bonds that connect the N-terminus to the core of the structure, XCL1 only has one. This simple difference in disulfide bonds changes the overall tertiary structure of XCL1 from other chemokines. There are two parts of the lymphotactin protein, structures Ltn10 and Ltn40, that folds into each other, which make it biologically active. This conformational change alters the binding structures on the chemokine. This understanding of the interfolding provides more of a basis to understanding to the lymphotactin kinetics. Biological significance The pair of XCL1 and XCR1 are known to be involved in cross-presentation, antigen uptake, and induction of innate as well as adaptive cytotoxic immunity. XCR1, the receptor for XCL1, is exclusively expressed in conventional dendritic cells. XCL1 is secreted by NK cells and by antigen-specific CD8+ T-cells, along with other chemokines including IFN-gamma. This process likely facilitates the cross-presentation of antigens by the dendritic cells. XCL1 is also known to increate T cells in joints that are effected with rheumatoid arthritis. They are also expressed on RA synovial lymphocytes. References Cytokines