| down promoter mutation | <molecular biology> A mutation (a change in base pair sequence) in a promoter region, this results in lower gene expression (less transcription of the gene occurs). (09 Oct 1997) |
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| down-regulation | <physiology> Development of a refractory or tolerant state consequent upon repeated administration of a pharmacologically or physiologically active substance. It is the process that decreases ligand and receptor interactions or reduces the responsiveness of a cell to a stimulus following first exposure. This is often accompanied by an initial decrease in affinity of receptors for the agent and a subsequent reduction in the number of available receptors expressed on the surface which can result from internalisation of the ligand:receptor complex or from decreased expression of the receptor. Classically the concept referred to hormone receptors but contemporary usage includes other cell surface receptors. (03 Jul 1999) |
| Down's syndrome | <genetics, syndrome> A congenital disorder in which a person is born with three copies of chromosome 21 (trisomy 21). Clinical features include moderate to severe mental retardation, slanting eyes, a broad short skull, broad hands and short fingers. Other congenital abnormalities include heart defects, oesophageal atresia and an increased incidence of acute lymphocytic leukaemia. Trisomy 21 can be detected in the first few months of pregnancy by amniocentesis. Risk factors include prior child with Down's syndrome and mothers who become pregnant after age 40. This disorder was formerly known as mongolism. Incidence: 1 in 1000 births. (09 Oct 1997) |
| down syndrome | <radiology> Trisomy 21, mongolism, atlantoaxial subluxation, Wormian bones, absent/small paranasal sinuses, endocardial cushion (AV canal) defects, aberrant right subclavian artery, 2 ossification centres for manubrium (90%), 11 or 13 ribs, duodenal atresia / stenosis, tracheo-oesophageal fistula, Hirschsprung disease (12 Dec 1998) |
| turn down ratio | The lowest load at which a boiler will operate efficiently as compared to the boiler's maximum design load. (05 Dec 1998) |
| let-down reflex | <neurology, paediatrics> An involuntary reflex during breast feeding which causes the milk to flow freely. See: milk-ejection reflex (22 Sep 2002) |