NK cell cytolytic function and cytokine production are regulated by a delicate balance between signaling through activating and inhibitory receptors Lanier 2000; Trinchieri 1989). Previous attention in the field has focused on MHC-recognizing receptors that are mostly inhibitory Long 1999). However, members of the CD2 subset of receptors do not recognize MHC molecules, but still play a major role in natural killer NK)-and T-cell functions. Two members of the CD2 subset, 2B4 CD244) and SLAM CD150), are involved in cellular activation such as lymphoproliferation, cytokine production, cytotoxicity, and invasiveness Aversa et al. 1997; Boles et al. 1999; Chuang et al. 2000; Mathew et al. 1993; Nakajima et al. 1999; Punnonen et al. 1997). Their cytoplasmic domains contain novel tyrosine motifs TxYxxI/V/A) differing from those observed in other NK-and T-cell receptors Boles et al. 1999; Cocks et al. 1995; Nakajima and Colonna 2000). CD84, another member of the CD2 subset, also contains the consensus tyrosine motif, but its function remains to be determined de la Fuente et al. 1997). Unlike many NK-cell receptors that bind MHC class I molecules as their ligands Long 1999), CD2 subfamily members bind homophilically to members of the CD2 subfamily Davis et al. 1998). CD2 binds CD58 and CD48 in the human and CD48 in the rodent Kato et al. 1992; van der Merwe et al. 1993, 1994). Additionally, SLAM is a self-ligand that activates T and B cells Punnonen et al. 1997; Mavaddat et al. 2000). CD48 is a high-affinity ligand for 2B4 Brown et al. 1998). Furthermore, CD48 is widely expressed in leukocytes and its soluble form is elevated in the blood of patients with lymphoid leukemias and arthritis Smith et al. 1997). The novel tyrosine motifs in SLAM and 2B4 interact with the src homology 2 SH2) domain of the SLAM-associated protein SAP)/SH2D1A which may regulate signaling of the receptor Sayos et al. 1998; Tangye et al. 1999). Mutations in the SH2 domain of SAP/SH2D1A have been identified as the genetic basis for X-linked lymphoproliferative disease XLP) Coffey et al. 1998; Nichols et al. 1998; Sayos et al. 1998). Recently, Parolini and co-workers 2000) reported that the 2B4 receptor transduces inhibitory signals in XLP patients when stimulated by CD48-positive B cells. These data suggest that the cause of XLP is complex and probably due to dysregulation of phosphorylation-dependent interactions at multiple sites, including receptors and cytoplasmic adaptor molecules. In addition to 2B4 and SLAM, other receptors that contain the unique tyrosine motif might also contribute to the immune dysregulation seen in XLP. Here we report the cloning and characterization of a new member of the CD2 subset of receptors, named CS1, that contains two of the unique tyrosine motifs. Dysregulated signaling via the CS1 receptor may also contribute to the phenotype of XLP. CS1 is localized to the long arm of human Chromosome Chr) 1 between CD48 and Ly-9 CD229). To identify receptors that contain the novel tyrosine motif, we searched the expressed sequence tag EST) database at GenBank http://www. ncbi. nlm. nih. gov) with the TblastN program versus a consensus sequence of human members of the CD2 subset. Several overlapping clones were identified and oligonucleotide primers CS1 F1, 59-cctcccatggtcctcctgtg-39; CS1 R1, 59-gagacttaggggagtgcactgctg-39) were designed to amplify a 363-bp fragment within the cytoplasmic domain. cDNA from a NK-cell library constructed in l phage by Dr. J. Houchins R & D Systems, Minneapolis, Minn., and kindly provided by Dr. A. Brooks, NIH, Bethesda, Md.) was successfully used as tem-