CTSD and Alzheimer disease: Through exploring the key Sphingolipid signaling pathway and Sphingolipid metabolism of gene expression, we found that the ASAH1, PIK3R1, PRKCB, and PSAPL1 lower the expression in AD, while CTSD, FCER1G, RAC2, SPTLC2, and NEU2 showed elevated expression (Figure 7A,B).