With regard to AD, most of the post-mortem studies have examined the level of ceramide and sphingosine, the pro-apoptotic precursors of sphingosine 1-phosphate, or enzymes responsible for their generation such as acid sphingomyelinase (which degrades sphingomyelin to ceramide) or acid ceramidase (which converts ceramide to sphingosine). Here, SMPD1 is linked to Alzheimer disease.