A bioactive sphingolipid; naturally occurring form of sphingomyelin; increased by approximately 35-fold in CCD-112 colon fibroblasts compared with HCT116 human colorectal carcinoma cells; has been used as an internal standard for the quantification of sphingolipid metabolites; a mixture of the D-erythro and L-threo isomers of C18 sphingomyelin