High Quality Content by WIKIPEDIA articles! X-linked lymphoproliferative disease is a lymphoproliferative disorder. There is a mutation on the X chromosome that has been found to be associated with a T and NK cell lymphoproliferative disorder. The mutation is on the long arm of the chromosome, at position 25, which is denoted as Xq25. At this position, there is a deletion in the SH2D1A gene, which codes for an SH2 domain on a signal transducing protein called SLAM-associated protein (SAP). The term SH2 domain stands for src-homology 2 domain, which is a three-dimensional domain structure of about 100 amino acid residues. These domains are present in many signaling proteins because they permit specific, non-covalent bonding to proteins that contain phosphotyrosines. The amino acid residues adjacent to the phosphotyrosine on the target protein are what determine the unique binding specificity.