ASAH1 and Alzheimer disease: Based upon the above findings on SM deacylase, as shown schematically in Figure 32, 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 (C31/C340) between the α- and β-subunits of aCDase could be impaired in AD skin, probably due to a point mutation of the aCDase proenzyme although no such point mutations are currently known; (2) breaking the S-S bond (C31/C340) could occur more easily in AD skin than in HC skin via unknown mechanisms.