Mutation of the mouse gene causes various types of abnormality in the development and functions of the nervous system. Many mutant animals show abnormal morphogenesis and behavior. The analysis of these mutants compared with those of the control mice yields much information about the development and differentiation of the nervous system. This chapter presents two examples of the results of such analyses: (1) mutations showing deficiencies in myelination and (2) mutant mice showing the deficiencies of P400 protein, a protein enriched in Purkinje cell, which was identified recently as an inositol 1,4,5-trisphosphate (InsP3)receptor. InsP3 receptor is a calcium-releasing channel located on the endoplasmic reticulum and is now a very important molecule for signal transduction cascade in the cell. The chapter provides an example of myelin-deficient mutation, shiverer, mld, and jimpy. Analysis of the mutation by producing chimera animals is very important because it enables to analyze the events occurring among cells. Analysis of jimpy has provided a new finding that myelin protein has another functional role such as factors important for differentiation.