The biochemical and genetic understanding of human disease has increased explosively in recent years. We now understand that most human disease is the result of interactions between genetic factors and environmental influences, and the combined influences are being identified in our society's most important and burdensome diseases, including cancer, diabetes, and degenerative disorders such as arthritis, Alzheimer's and Parkinson disease. When mutant genes combine with environmental factors to produce a cascade of malfunctions, treatment may take one of many different approaches. This book describes the range of disorders and the treatments which have become available for them as a result of our increased understanding. These treatments range from classical approaches such as the supply of a missing product of enzyme activity, through enzyme or cofactor replacement, to the exciting possibility of therapy directed specifically to the underlying defect, the mutant gene itself.