Transmembrane adapter proteins (TRAPs) represent a relatively new and unique group of signalling molecules in hematopoetic cells. They differ from other signalling proteins as they lack any enzymatic or transcriptional activity, instead they possesses multiple tyrosine-based signalling motifs (TBSMs). Triggering of immunoreceptors induces tyrosine phosphorylation of these motifs by members of the Src-, Syk- or Tec-family of protein tyrosine kinases thus enabling the TRAPs to recruit cytosolic adapter and/or effector molecules via their SH2-domains into close proximity to the immunoreceptors, a position from which they can coordinate and modulate signal transduction pathways important for lymphocyte function.