Over the years, the classification of metabolic diseases has extended beyond traditional pathways involved in fuel metabolism to include disorders such as lysosomal storage diseases or connective tissue diseases. Thus, metabolic diseases really reflect disorders of cell biology. For example, lysosomal storage diseases (Chap. 361) result from a variety of defects, usually in a lysosomal enzyme, causing accumulation of a substrate within the lysosome. Certain lipodystrophies and cardiomyopathies can be caused by mutations in lamin A, a structural protein in the nuclear envelope. Membrane defects (Chap. 365), usually involving transporters of amino acids, sugars, or ions, cause disorders such as cystinuria, Hartnup's disease, or Wilson's disease. Connective tissue diseases (Chap. 363) frequently involve defects in collagen synthesis or structure (osteogenesis imperfecta, Ehlers-Danlos syndrome, Alport's syndrome) or in other extracellular matrix structural proteins such as fibrillin (Marfan syndrome). Many metabolic disorders originate from defects in enzymes involved in the synthesis or degradation of amino acids, carbohydrates, lipids, purines, or pyrimidines. (Chaps. 359, 362, 364). Lipoprotein disorders (Chap. 356) are caused by defects in a wide array of cellular pathways including membrane receptors (LDL-R), enzyme defects (lipoprotein lipase), carrier proteins (apoB100), or transporters (ABCA1). In some instances, metabolic abnormalities induce compensatory physiologic responses that reflect the interactions of multiple different metabolic pathways. For example, the metabolic syndrome (Chap. 242) includes a constellation of clinical features [central obesity, hypertriglyceridemia, low high-density lipoprotein (HDL) cholesterol, hyperglycemia, and hypertension]. It likely has multiple genetic and environmental origins. Cushing's syndrome reflects the metabolic effects of excess cortisol on multiple tissues (Chap. 342).