pcdh18 contains a motif (centered at Y842) shared with Src kinases (QGQYQP) that is required for the inhibitory phenotype. Thus, pcdh18 is a novel activation marker of CD8(+) memory T cells that can function as an inhibitory signaling receptor and restrict the effector phase. Mouse Pcdh18 (mPcdh18), which consists of four exons similar to other protocadherin family members, maps to chromosome 3. The deduced amino acid sequence of mPcdh18 contains six extracellular cadherin motifs, a single transmembrane region, and a large intracellular domain. The site of Disabled-1 (Dab1) interaction was localized to the C-terminal 243 residues of mPcdh18. Expression analyses revealed that mPcdh18 is present in a variety of tissues in the embryo, but in adult mice, it is primarily expressed in the lung and kidney. In the embryonic brain, mPcdh18 expression is temporally and spatially regulated.