Given the clinical observation that some patients with PAX3 mutations are hydrocephalic [12,13], our characterization of the Pax3 conditional and compound haploinsufficient mutant mouse models has provided an initial molecular basis to begin to understand how genetic, and possibly environmental, factors could affect expression of Pax3 and/or Pax7, and impact the prevalence of congenital hydrocephalus. This evidence concerns the gene PAX3 and congenital hydrocephalus.