Daily function of cells involves many biochemical pathways that continuously monitor and balance energy homeostasis. Disruption of cellular metabolism can lead to diseases that affect most systems in the body. A hallmark of cancer, for example, is a restructuring of normal cellular metabolism to meet the energetic and stress demands inherent in rapid cell growth. Diseases that involve inborn errors of metabolism (IEM’s) include phenylketonuria, Niemann-Pick disease, and glycogen storage diseases.