BACE1 and Alzheimer disease: The following points need to be further clarified: (1) whether the changes in miRNA content in different brain regions, cerebrospinal fluid, and serum are related and whether the changes are AD-specific; (2) some miRNAs may show opposing trends during different stages of AD, and how to effectively divide the boundaries remains a major challenge; (3) because miRNAs exert a multitarget effect, and many key factors of AD are influenced by more than one species of miRNA: this is exemplified by BACE1 which is regulated by at least 10 more species of miRNA.