Soluble Aβ40 and 42 are well known to be decreased in CSF18, 19 and increased in the peripheral blood,30 which represent changes that are used for diagnosing AD.31, 32 We found here that the p75NTR-ECD level is also reduced in the CSF and increased in the serum, and that both phenomena have diagnostic significance and are sensitive and specific as Aβ. This evidence concerns the gene NGFR and Alzheimer disease.