Mice with more severe phenotypes display exencephaly and die in utero .
Defects are sensitive to a variety of environmental factors.
Targeted mutation of Apob . Model for human hypobetalipoproteinemia. Homozygotes characterized by exencephalus of varying severity that appears to be the result of abnormal supporting mesenchyme. Targeted mutation in Csk . Homozygotes deficient for CSK, a negative regulator of Src family tyrosine kinases, die in utero .