3p deletion syndrome is a rare contiguous-gene disorder involving the loss of the telomeric portion of the short arm of chromosome 3 and characterized by developmental delay, growth retardation, and dysmorphic features. All reported cases have involved, at a minimum, the deletion of chromosome 3 telomeric to the band 3p25.3. Despite the presence of several genes in this region that are involved in neural development, a causative relationship between a particular transcript and the observed clinical manifestations has remained elusive. We have identified a child with characteristic physical features of 3p deletion syndrome and both verbal and nonverbal developmental delay who carries a de novo balanced translocation involving chromosomes 3 and 10. Fine mapping of this rearrangement demonstrates that the translocation breakpoint on chromosome 3 falls within the recently identified minimal candidate region for 3p deletion syndrome and disrupts the Contactin 4 (CNTN4) mRNA transcript at 3p26.2-3p26.3. This transcript (also known as BIG-2) is a member of the immunoglobulin super family of neuronal cell adhesion molecules involved in axon growth, guidance, and fasciculation in the central nervous system (CNS). Our results demonstrate the association of CNTN4 disruption with the 3p deletion syndrome phenotype and strongly suggest a causal relationship. These findings point to an important role for CNTN4 in normal and abnormal CNS development.