SMPD1 and Alzheimer disease: SM deacylase competes with aSMase and BGCase to hydrolyze their common substrates, SM or GCer, to yield their lysoforms sphingosylphosphorylcholine (SPC) or glucosylsphingosine (GS), respectively, instead of ceramide, that results in the ceramide deficiency in AD skin, independent of the activities of aSMase and BGCase.