Based upon the above findings, we hypothesize two possible causative biological factors that underlie the expression of SM deacylase in AD skin as follows: (1) The formation of the S-S bond between the α- and β-subunits of aCDase could be impaired in AD skin, presumably due to a possible point mutation of the aCDase proenzyme, although no such point mutations are currently known to exist. The gene discussed is ASAH1; the disease is Alzheimer disease.