The function of CERT1 (ceramide transporter 1, Ensembl gene identifier ENSG00000113163) is as follows. Shelters ceramides inside its steroidogenic acute regulatory lipid transfer (START) domain and mediates their intracellular trafficking in a non-vesicular manner from the endoplasmic reticulum to the Golgi apparatus for conversion to sphingomyelin (PubMed:14685229, PubMed:15596449, PubMed:17392267, PubMed:17591919, PubMed:18184806, PubMed:20036255). Efficiently transfers ceramide molecules having long- chain fatty chains, but not those with very long acyl chains (PubMed:15596449, PubMed:18184806). Capable of transferring diacylglycerol, although with very low efficiency (PubMed:18184806). {ECO:0000269|PubMed:14685229, ECO:0000269|PubMed:15596449, ECO:0000269|PubMed:17392267, ECO:0000269|PubMed:17591919, ECO:0000269|PubMed:18184806, ECO:0000269|PubMed:20036255}.