SMPD1 and Niemann-Pick disease: Since a barrier recovery pattern with a significant delay only at 1 day after tape-stripping in AD skin compared to HC skin was almost identical to that as seen in aSMase-deficient Niemann–Pick disease, we thought it likely that the impaired homeostasis of a ceramide-generating process other than the known sphingolipid metabolic enzymes may be associated with the continued abnormality of barrier and water reservoir functions due to the ceramide deficiency in nonlesional AD skin.