The CSF levels of both FL-Ng and CT-Ng were significantly elevated in patients with MCI-AD (5.69 ng/L, P < 0.00001 and 566 ng/L, P < 0.00001, respectively) and AD (6.02 ng/L, P < 0.00001 and 452 ng/L, P = 0.00001, respectively) compared with controls (0.644 ng/L and 145 ng/L, respectively) (Fig. 1). Here, NRGN is linked to Alzheimer disease.