The amyloid precursor protein (APP) is the source of amyloid-beta (A beta) peptide, produced via its sequential cleavage beta- and gamma-secretases. Various biophysical forms of A beta (and the mutations of APP which results in their elevated levels) have been implicated in the etiology and early onset of Alzheimer's disease. APP's evolutionary conservation and the existence of APP-like isoforms (APLP1 and APLP2) which lack the A beta sequence, however, suggest that these might have important physiological functions that are unrelated to A beta production. Soluble N-terminal fragments of APP have been known to be neuroprotective, and the interaction of its cytoplasmic C-terminus with a myriad of proteins associates it with diverse processes such as axonal transport and transcriptional regulation. The notion for an essential postnatal function of APP has been demonstrated genetically, as mice deficient in both APP and APLP2 or all three APP isoforms exhibit early postnatal lethality and neuroanatomical abnormalities. Recent findings have also brought to light two possible functions of the APP family in the brain-regulation of neural progenitor cell proliferation and axonal Outgrowth after injury. Interestingly, these two apparently related neurogenic/neuroregenerative functions of APP involve two separate domains of the molecule. (c) 2005 Elsevier Inc. All rights reserved.