Webster's New World Medical Dictionary

reversal of organs, total
Complete transposi-
tion of the thoracic and abdominal organs from
right to left, placing the heart in the right side of the
chest, and so on. Organs appear as if in mirror
image when examined or X-rayed. Total reversal of
organs has been estimated to occur in about 1 in
6,000 to 8,000 births. It also occurs in a rare,
abnormal congenital condition called Kartagener
syndrome. Also known as situs inversus totalis. See
also 
dextrocardia; Kartagener syndrome.
reverse genetics
In molecular genetics, identify-
ing genes purely on the basis of their position in the
genome, with no knowledge whatsoever of the gene
product. In classic genetics, the traditional
approach was to find a gene product and then try to
identify the gene itself. Also known as positional
cloning.
reverse transcriptase
An enzyme that permits
DNA to be made, using RNA as the template. A
retrovirus, such as the HIV virus, can propagate
itself by converting its RNA into DNA with reverse
transcriptase.
Reye’s syndrome
A sudden and sometimes fatal
disease of the brain (encephalopathy) that is accom-
panied by degeneration of the liver. Reye’s syndrome
usually occurs in children between the ages of 4 and
12, comes after infection with chickenpox (vari-
cella) or an influenza-type illness, and is associated
with taking medications that contain aspirin. A child
with Reye’s syndrome first tends to be unusually
quiet, lethargic (stuporous), and sleepy. Vomiting
may occur. In the second stage, the lethargy deep-
ens, and the child becomes confused, combative,
and delirious. This stage is followed by decreasing
consciousness, coma, seizures, and eventually death.
Early diagnosis and control of the increased
intracranial pressure can prevent death or brain
damage. Preventing Reye’s syndrome is the reason
why physicians no longer recommend giving chil-
dren aspirin for fever or for any other reason.