UGCG and neoplasm: Ceramidase-derived sphingosine may also be acylated by ceramide synthase to form a stereochemically variant L-threo ceramide that cannot be glucosylated by glycosylceramide synthase (GCS) to generate glycosylceramide (reviewed by Reynolds et al. 2004) and this has been shown to synergistically enhance tumor cell killing by the ceramide-generating retinoid fenretidine (Maurer et al. 2000).