Human insulin, the hormone that regulates blood glucose, is a small protein made of 51 amino acids arranged in two polypeptide chains: chain A, of 21 amino acids, and chain B, of 30 amino acids, held together by disulphide bonds. The cell first makes insulin as a single, longer precursor molecule (proinsulin), which includes an extra connecting segment, the C-peptide, joining what will become the A and B chains; this C-peptide is enzymatically cut out during processing, leaving behind the final, active, two-chain insulin molecule.
A vaccine is a biological preparation — a killed/inactivated pathogen, a weakened (attenuated) pathogen, or a harmless component of one — given to stimulate the immune system into producing protective antibodies and memory cells against a specific disease, without ever causing the disease itself. Vaccines fall into a few broad types: attenuated whole-agent vaccines, which use a live but deliberately weakened pathogen (e.g. the traditional oral polio vaccine, or the BCG vaccine for tuberculosis); inactivated (killed) whole-agent vaccines, which use a pathogen that has been killed but still displays its surface antigens (e.g. the injectable Salk polio vaccine); and toxoid vaccines, which use a chemically or heat-inactivated version of a toxin the pathogen produces, rather than the pathogen itself (e.g. tetanus toxoid, diphtheria toxoid).
Gene therapy is the correction of a genetic disorder by introducing a normal, functioning copy of a gene into a patient's cells, to compensate for one that is defective or missing. It is classified along two independent lines. By the cells targeted: germ-line gene therapy corrects the gene in reproductive (germ) cells, so the correction would be passed on to offspring — not currently practised in humans, for ethical and safety reasons — while somatic-line gene therapy corrects the gene only in the patient's ordinary body cells, so the correction affects only that patient and is not inherited. By where the correction is carried out: ex-vivo gene therapy removes cells from the patient, corrects them in the laboratory, and reintroduces them, while in-vivo gene therapy delivers the corrective gene directly into the patient's body, often using a viral vector. The best-known success story of gene therapy is the treatment of ADA (adenosine deaminase) deficiency, a cause of Severe Combined Immuno-Deficiency (SCID), in which a functional ADA gene is introduced into the patient's own lymphocytes. …