Endothelial cells preferentially manifest 20-fold enrichment of a non-lysosomal secretory form of ASMase[9] that renders them particularly vulnerable to radiation-induced ASMase-mediated generation of the pro-apoptotic second messenger ceramide[10], and evidence indicates ceramide-mediated apoptosis is causative of the vascular component of tumor response to single dose radiotherapy[10]. This evidence concerns the gene SMPD1 and neoplasm.