Congenital hydrocephalus (CH) is a poorly understood life-threatening condition caused by cerebrospinal fluid (CSF) build-up in the brain, the onset of which can occur as early as in utero or in neonates. Of the handful of genes associated with CH, the most commonly mutated gene is L1, which encodes the L1 cell adhesion molecule. How loss of L1 results in hydrocephalus is not known, in large part because of complex genetic interactions that can modify the expressivity and penetrance of L1-associated hydrocephalus. Through their studies, this group identified synergistic genetic interactions between L1 and Ras-Erk pathways they hypothesize underlie a percentage of congenital hydrocephalus. To test this hypothesis, the researchers will conduct a pilot screen of whole exome sequences of six L1 patients and their unaffected parents to identify second site mutations components that function in the L1 pathway as well as in genes that result in overactive Ras-Erk signaling.