The SH2- and SH3-containing adaptor molecules serve to recruit cytosolic signal-transducing molecules to the activated receptor tyrosine kinases. In this study, we report the molecular cloning of a novel adaptor-like protein,Grap-2(Grb-2relatedadaptorprotein2), using the multisubstrate docking proteinGab-1as bait in the yeast two-hybrid system. Sequence analysis revealed thatGrap-2contains a SH3–SH2–SH3 structure that has a high degree of sequence homology to those of theGrb-2andGrapadaptor molecules. However, unlike inGrapandGrb-2,the SH2 and the C-terminal SH3 domains ofGrap-2are separated by a 120-amino-acid glutamine-rich sequence that shows no apparent homology to any known molecule or structural motif. The C-terminal SH3 domain ofGrap-2alone is sufficient to bind toGab-1.Furthermore, Northern blot analysis demonstrated thatGrap-2has two major transcripts of 1.4 and 4.0 kb that can only be detected in tissues rich in leukocytes and in two leukemia cell lines. This highly restricted pattern of expression suggests thatGrap-2may participate in leukocyte-specific protein tyrosine kinase signaling.