Children with brain injuries that are apparently similar in etiology, severity, and clinical management have striking differences in outcomes. There is increasing evidence that genetic factors may influence brain injury severity and outcome. Apolipoprotein E (apoE) is a lipid transport protein abundantly present in cells in the brain. In addition to transporting lipids, apoE has other functions necessary for cell maintenance and repair. The gene for apoE exists in three alleles (ε2, ε3, and ε4). Strittmatter and colleagues found APOE ε4 to be an important predisposing factor for the development of Alzheimer's disease. Subsequently, the ε4 allele was shown to convey vulnerability to worse outcome after traumatic brain injury and other neurological insults in adults.