Introduction

As discussed elsewhere (Przedborski et al., 2003), the term “neurodegenerative disease” refers to a group of neurological disorders with heterogeneous clinical and pathological expressions. These diseases are all characterized by a loss of specific subpopulations of neurons confined to functional anatomic systems, arising in most cases for unknown reasons and progressing in a relentless manner. Among the variety of neurodegenerative disorders, the lion's share of attention has been given only to a handful, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS). So far, the most consistent risk factor for developing a neurodegenerative disorder, especially AD and PD, is increasing age. Over the past century the growth rate of the population age 65 and beyond in the industrialized countries has far exceeded that of the population as a whole. Thus it can be anticipated that, over the next generations, the proportion of elderly citizens will double and, with this, the number of individuals suffering from a neurodegenerative disorder. This prediction is at the centre of the growing concerns from the medical community and from legislators, as one can easily foresee a dramatic increase in the emotional, physical, and financial burden on patients, caregivers, and society related to these disabling illnesses. The problem is made worse by the fact that, although to date several approved drugs do, to some extent, alleviate symptoms of several neurodegenerative diseases, their chronic use is often associated with debilitating side effects, and none seems to stop the progression of the degenerative processes.