DAPP1 (dual adaptor of phosphotyrosine and 3-phosphoinositides 1) is a 280 amino acid adapter protein containing an N-terminal myristoylation site, an SH2 domain, and a C-terminal pleckstrin homology (PH) domain 1. It binds with high affinity to phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate, serving as a molecular bridge between tyrosine-phosphorylated proteins and phosphoinositide signaling 1. DAPP1 functions as a B-cell-associated adapter regulating B-cell antigen receptor signaling downstream of PI3K 2. At the plasma membrane of activated lymphocytes, DAPP1 assembles protein complexes that promote activation of small GTPases including Rac and Rap, facilitating integrin activation and lymphocyte adhesion 2. DAPP1 is also recognized as a transcriptional target of T-bet in CD21lo B cell differentiation 3. Beyond lymphocyte function, DAPP1 acts as a negative regulator of platelet activation; DAPP1-deficient mice show enhanced thrombus formation and increased granule secretion 4. Genomic studies implicate DAPP1 variants in airway hyperreactivity responses to diesel exhaust particle exposure 5 and associate DAPP1 expression changes with treatment resistance in ulcerative colitis and systemic lupus erythematosus-like disease 67.