Defects in neural tube formation are among the most common malformations leading to infant mortality. Although numerous genetic loci appear to contribute to the defects observed in humans and in animal model systems, few of the genes involved have been characterized at the molecular level. Mice lacking the p53 tumour suppressor gene are predisposed to tumours, but the viability of these animals indicates that p53 function is not essential for embryonic development. Here, we demonstrate that a fraction of p53–deficient embryos in fact do not develop normally. These animals display defects in neural tube closure resulting in an overgrowth of neural tissue in the region of the mid–brain, a condition known as exencephaly.