We report for the first time monozygous twins with a microdeletion syndrome involving genes coding for Bruton's tyrosine kinase (Btk) and deafness-dystonia peptide 1 (DDP1), and two other genes. Apart from its essential role in B cell development, Btk is indicated to affect signaling mediated by toll like receptors (TLRs) and development of dendritic cells (DCs) but results are conflictive. The twins revealed normal numbers of plasmacytoid and myeloid DCs (pDCs and mDCs). Moreover, BTK null cells from these patients exhibited robust responses to TLR agonists, normal natural killer (NK) cell activity, and normal pDC functions. CONCLUSION: Our results do not indicate the essential role of Btk in TLR signaling and DC development.