For example, the circRNA HDAC9 (circHDAC9) (best transcript: HDAC9, chr7:18684293-18,688,306) functions as a miR-138 sponge and is present at significantly lower levels in the serum of both patients with MCI and AD, and its levels are also reduced in mouse and cell models of AD (Lu et al., 2019). Here, HDAC9 is linked to Alzheimer disease.