SMPD1 and major depressive disorder: Once the source of the enzymatic activity, i.e., the corresponding gene(s), has been determined, analysis of the promoter(s), splice variants, and regulation could shed further light onto the complex sphingolipid pathway processed similarly as observed for the splice variants of the ASM gene SMPD1 [49], and their alteration in patients with major depressive disorder [50,51].