Neurexin 1 (NRXN1) is a cell adhesion protein, the normal function of which is critical for effective neurotransmission. It forms a trans-synaptic complex in the central nervous system with neuroligin. There has been one case in the literature of a patient with a heterozygous deletion in NRXN1 on one allele and a nonsense mutation on the other allele, reported to have a Pitt Hopkins-like phenotype. We report on two daughters of healthy, nonconsanguineous, Caucasian parents with biallelic NRXN1 deletions identified by array CGH. The children presented with severe early onset epilepsy, profound developmental delay, gastroesophageal reflux disease, constipation, and early onset puberty. Our report confirms that biallelic NRXN1 mutations result in a severe recessive mental retardation syndrome and broadens the range of phenotypes associated with this gene. © 2011 Wiley-Periodicals, Inc.