Identification of the genetic basis of rare monogenic diseases can provide insights into disease mechanisms, biological pathways and potential therapeutic targets. Exome sequencing has proven to be a powerful, efficient strategy for the identification of the genes underlying rare Mendelian disorders. The genetic defect behind the lethal malformation syndrome under study is known in some cases and has proven useful in revealing the primary underlying defect. We have six typical unrelated families with no known mutation. The aim of this study is to identify the locus associated with this lethal malformation syndrome in these families using exome sequencing.