In the course of systematic screening of the X-chromosome coding sequences in 250 families with nonsyndromic X-linked mental retardation (XLMR), two families were identified with truncating mutations in BRWD3, a gene encoding a bromodomain and WD-repeat domain–containing protein. In both families, the mutation segregates with the phenotype in affected males. Affected males have macrocephaly with a prominent forehead, large cupped ears, and mild-to-moderate intellectual disability. No truncating variants were found in 520 control X chromosomes. BRWD3 is therefore a new gene implicated in the etiology of XLMR associated with macrocephaly and may cause disease by altering intracellular signaling pathways affecting cellular proliferation. ; Michael Field, Patrick S. Tarpey, Raffaella Smith, Sarah Edkins, Sarah O’Meara, Claire Stevens, Calli Tofts, Jon Teague, Adam Butler, Ed Dicks, Syd Barthorpe, Gemma Buck, Jennifer Cole, Kristian Gray, Kelly Halliday, Katy Hills, Andrew Jenkinson, David Jones, Andrew Menzies, Tatiana Mironenko, Janet Perry, Keiran Raine, David Richardson, Rebecca Shepherd, Alexandra Small, Jennifer Varian, Sofie West, Sara Widaa, Uma Mallya, Richard Wooster, Jenny Moon, Ying Luo, Helen Hughes, Marie Shaw, Kathryn L. Friend, Mark Corbett, Gillian Turner, Michael Partington, John Mulley, Martin Bobrow, Charles Schwartz, Roger Stevenson, Jozef Gecz, Michael R. Stratton, P. Andrew Futreal, F. Lucy Raymond