It’, Trench Anderson 
example, the placing of an additional growth hormone gene into a 
normal child. 
(4) Eugenic genetic engineering: this is defined as the attempt to 
alter or ‘improve’ complex human traits, each of which is coded by 
a large number of genes: for example, personality, intelligence, 
character, formation of body organs, and so on. 
SOMATIC CELL GENE THERAPY 
There are many examples of genes which, when defective, produce 
serious or lethal disease in a patient. Gene therapy should be bene- 
ficial primarily for the replacement of a detective or missing enzyme 
or protein that must function inside the cell that makes it, or of a 
deficient circulating protein whose level does not need to be exactly 
regulated (for example, blood clotting factor VIII which is deficient 
in hemophilia). Early attempts at gene therapy will almost certainly 
be done with genes for enzymes that have a simple ‘always-on’ type 
of regulation. (For a technical discussion of the state-of-the-art of 
somatic cell gene therapy, together with extensive references, sec 
Anderson. 1984.) 
initial Candidates for Gene Therapy 
The most likely genes to be used in the first experiments on human 
gene therapy are: hypoxanthinc-guanine phosphoribosyl transferase 
(HPRT), the absence of which results in Lesch-Nyhan disease (a 
severe neurological disorder that includes uncontrollable self-mutila- 
tion); adenosine deaminase (ADA), the absence of which results in 
severe combined immunodeficiency disease (in which children have 
a greatly weakened resistance to infection and cannot survive the 
usual childhood diseases); and purine nucleoside phosphorylase 
(PNP), the absence of which results in another form of severe im- 
munodeficiency disease. For all three, the clinical syndrome is pro- 
foundly debilitating. The disorder in each is found in the patient’s 
bone marrow (although the severe central nervous system manifesta- 
tions of Lesch-Nyhan disease are due to absence of HPRT in brain 
cells and probably cannot be corrected with current techniques). In 
all three, there is no, or minimal, detectable enzyme in marrow cells 
from patients who have no copies of the normal gene. In these 
patients, the production of a small percentage of the normal enzyme 
level should be beneficial and a mild overproduction of enzyme 
Recombinant DNA Research, Volume 12 
[319] 
