The clinical features of loss of ARHGAP4 function remain unclear despite several reports of different patterns of deletions inactivating different functional regions of the protein. The protein encoded by ARHGAP4 is thought to function as a Rho GTPase activating protein. Characterization of the genetic defect causing X-linked nephrogenic diabetes insipidus (NDI) and intellectual disability in two dizygotic twin brothers revealed a novel contiguous deletion of 17,905 bp encompassing the entire AVPR2 gene and extending into intron 7 of the ARHGAP4 gene. Examination of their mother showed that she was a carrier of this deletion. An attempt was made to distinguish the putative clinical signs of an ARHGAP4 deletion from the well-defined phenotype of X-linked NDI caused by an AVPR2 gene deletion. By reviewing all characterized deletions encompassing ARHGAP4, we reconsider the potential role of ARHGAP4 in cognition. ; Lingli Huang, Gemma Poke, Jozef Gecz and Kate Gibson