SCD and Alzheimer disease: Further, other sphingolipid metabolic enzymes that function upstream of the hydrolysis of GCer and SM in the epidermis, such as serine-palmitoyl transferase (SPT), stearoyl CoA desaturase (SCD), ceramide synthases (CERS) 1–5, GCer synthase (GCERS), alkyl chain elongation enzymes and SM synthase (SMS), have never been reported to be implicated in the ceramide deficiency in uninflamed nonlesional AD skin [52].