The MARCKS protein is a widely distributed cellular substrate for protein kinase C. It is a myristoylprotein that binds calmodulin and actin in a manner reversible by protein kinase C-dependent phosphorylation. It is also highly expressed in nervous tissue, particularly during development. To evaluate a possible developmental role for MARCKS, we disrupted its gene in mice by using the techniques of homologous recombination. Pups homozygous for the disrupted allele lacked detectable MARCKS mRNA and protein. All MARCKS-deficient pups died before or within a few hours of birth. Twenty-five percent had exencephaly and 19% had omphalocele (normal frequencies, < 1%), indicating high frequencies of midline defects, particularly in cranial neurulation. Nonexencephalic MARCKS-deficient pups had agenesis of the corpus callosum and other forebrain commissures, as well as failure of fusion of the cerebral hemispheres. All MARCKS-deficient pups also displayed characteristic lamination abnormalities of the cortex and retina. These studies suggest that MARCKS plays a vital role in the normal developmental processes of neurulation, hemisphere fusion, forebrain commissure formation, and formation of cortical and retinal laminations. We conclude that MARCKS is necessary for normal mouse brain development and postnatal survival.