The gene defect responsible for the X‐linked lymphoproliferative disease (XLP) is associated with an impaired control of Epstein‐Barr virus (EBV) infection. The gene has been recently identified and the encoded protein (designated SH2D1A, DSHP or SAP) was characterized. It is a 128 amino acid (aa) protein, containing a single Src homology 2 (SH2) domain. It interacts with signaling lymphocytic activation molecule (SLAM) expressed on the surface of activated T and B cells. We show that activated T, but not activated B, cells express the SH2D1A protein. NK cells express the protein as well. Tumor lines originating from B, T or NK cells exhibited similar SH2D1A protein expression as the corresponding normal cells, with some notable exceptions. EBV‐carrying, tumor phenotype representative (type I), but not EBV‐carrying lymphoblastoid cell line (LCL)‐like (type III) or EBV‐negative Burkitt lymphoma (BL) lines expressed SH2D1A. The phenotypic switch from type I to type III in the EBV‐carrying BL line Mutu was associated with a down‐regulation of SH2D1A and up‐regulation of SLAM. In contrast to normal ex vivo and long‐term activated NK cells, 2 of 3 NK leukemia lines expressed SLAM. All 3 lines expressed SH2D1A, like their normal counterparts. Int. J. Cancer 88:439–447, 2000. © 2000 Wiley‐Liss, Inc.