NGF and Alzheimer disease: Although for many years it was hypothesized that cholinotrophic basal forebrain cortical and hippocampal projection neurons degenerate due to loss of NGF in AD (Hefti and Mash, 1989; Tuszynski et al., 1990; Smith et al., 1999), studies reported unchanged (Goedert et al., 1989; Allen et al., 1991; Murase et al., 1993; Jette et al., 1994), decreased (Hellweg et al., 1998) or increased (Crutcher et al., 1993; Scott et al., 1995; Fahnestock et al., 1996; Narisawa-Saito et al., 1996; Hellweg et al., 1998; Hock et al., 2000) NGF levels using tissue from severe AD subjects.