The Insulin-like Growth Factors (IGF) – I and II are bound to specific binding proteins in circulation(IGFBP). Until today seven different proteins have been identified IGFBP-1 to 7. IGF bioavailability, transport and storage is regulated or facilitated by these binding proteins which areexpressed differentially according physiological and developmental requirements. The most abundantIGFBP in circulation is IGFBP-3. Together with IGFBP-5 it is able to form the so called ternary complexwith IGF and the acid-labile subunit (ALS). In the circulation nearly all IGF is bound in this ternarycomplex and thus not able to cross the endothelial barrier. Only very small amounts of IGF or IGFBP-3exist outside this complex. The acid-labile subunit is an important part of the IGF-storagemechanism in circulation. In ALS deficiency or in ALS knock-out mice the concentration of IGF andIGFBP-3 in the circulation is significantly decreased resulting in impaired growth.The acid-labile Subunit, is a synthesized as propeptide of 605 amino acids. The signal peptide,necessary for ALS secretion (AA 1-27) cleaved off enduring the transport process (Swiss-Prot P35858Version 82). The mature protein consists of 578 amino acids and contains about 20 leucin richsequence repeats. Beside the leucin-rich repeats several potential N-linked glycosylation sides havebeen described. Miller BS et al. were able to demonstrate that incomplete glycolsylation of IGFs, ALSand IGFBP-3 results in a decreased serum concentration of these proteins. Oral mannose therapyresulted in a partial normalization of the glycosylation pattern and went along with improved growth.Mutations in or the complete knock out of the ALS gene result in IGF / IGFBP-3 deficiency andtherewith in distribution of growth. Beside growth also other endocrine axes may be involved. Inprimary ALS deficiency hyperinsulinemia could be observed. Further, the HGH-IGF-IGFBPsystemseems to be of relevance in coronary disease.The first ALS immunoassay was described by Baxter RC in 1990 [6]. By this in-houseradioimmunoassay it was shown that ALS is present in high concentrations in serum (50µg/mL) ofhealthy humans. But not detectable in other body fluids like amniotic fluid, cerebrospinal fluid or seminal plasma – in spite of the fact that these body fluids contain high level IGFBP-3.