4-Trimethylammoniobutanoic acid, also known as gamma-butyrobetaine (GBB) or 3-dehydroxycarnitine, is a highly water-soluble derivative of gamma-aminobutyric acid (GABA). It is also a precursor of L-carnitine. It is a substrate of gamma butyrobetaine hydroxylase/dioxygenase (also known as BBOX) which catalyzes the formation of L-carnitine from gamma-butyrobetaine, the last step in the L-carnitine biosynthesis pathway. Carnitine is essential for the transport of activated fatty acids across the mitochondrial membrane during mitochondrial beta-oxidation. Numerous disorders have been described that lead to disturbances in energy production and in intermediary metabolism which are characterized by the production and excretion of unusual acylcarnitines. A mutation in the gene coding for carnitine-acylcarnitine translocase, or the OCTN2 transporter aetiologically, causes a carnitine deficiency that results in poor intestinal absorption of dietary L-carnitine, impaired reabsorption by the kidney, and increased urinary loss. Determination of the qualitative pattern of acylcarnitines can be of diagnostic and therapeutic importance. The betaine structure of carnitine requires special analytical procedures for recording. The ionic nature of L-carnitine causes a high water solubility which decreases with increasing chain length of the ester group in the acylcarnitines. Therefore, the distribution of L-carnitine and acylcarnitines in various organs is defined by their function and their physicochemical properties as well. High-performance liquid chromatography (HPLC) permits screening for free and total carnitine, as well as complete quantitative acylcarnitine determination, including the long-chain acylcarnitine profile , 136(8), 1279-1291., Int J Mass Spectrom. 1999;188:39-52.).