Natural killer cells (NK cells) belong to lymphocytes, besides more familiar B and T-lymphocytes. They comprise 5-10% of lymphocytes in blood and their role in the immune system is to discover and kill cells with cancer and cells infected by viruses. NK cells have a number of receptors on their surface, which are used for contact with other cells and for initiation of the cytotoxic response. Protein Clr-g, a target of this structural study, is a part of immune system of mouse. It is a ligand of NK receptor NKR-P1F. Clr-g occurs in dendritic cells and macrophages. The extracellular part of Clr-g was expressed, purified and crystallized and its structure was solved at 1.95 Å resolution using X-ray diffraction data measured at Bessy II of the Helmholtz Zentrum Berlin. The overall fold of mouse Clr-g is the fold typical of C-type lectin like proteins. Mouse Clr-g forms dimers and is most similar to human CD69. An interesting crystal contact was found in the crystal structure: N-terminus of the extracellular part of Clr-g binds to neighbour dimer in the crystal, and thus shows feasibility of peptide binding into the central pocket of the dimer. Mutual orientation of monomers in the dimer is slightly different than in the CD69 structure. However, moderate variability in orientations of monomers was found also among single structures of CD69. It seems that this difference gives testimony about flexibility in dimer formation and not about differences between mouse Clr-g and human CD69. Electrostatic potential was computed for several proteins structurally and functionally related to mouse Clr-g and surprisingly big differences were found.