Glycosphingolipids are organizational building blocks of plasma membranes that participate in key cellular functions, such as signaling and cell-to-cell interactions. Glucosylceramide synthase--encoded by the Ugcg gene--controls the first committed step in the major pathway of glycosphingolipid synthesis. Global disruption of the Ugcg gene in mice is lethal during gastrulation. We have now established a Ugcg allele flanked by loxP sites (floxed). When cre recombinase was expressed in the nervous system under control of the nestin promoter, the floxed gene underwent recombination, resulting in a substantial reduction of Ugcg expression and of glycosphingolipid ganglio-series levels. The mice deficient in Ugcg expression in the nervous system show a striking loss of Purkinje cells and abnormal neurologic behavior. The floxed Ugcg allele will facilitate analysis of the function of glycosphingolipids in development, physiology, and in diseases such as diabetes and cancer.