10 
of a metal loth ione in prcno tor and the structural gene for growth hormone 
releasing factor. This fusion gene also stimulates growth of mice but by a 
different mechanism through elevation of endogenous growth hormone production. 
Sane of the side effects of excess growth hormone production were alleviated 
by this strategy. 
Mr. Capron asked whether microinjected transgenic animals which do not pass 
the foreign gene to their offspring have been observed. Dr. Scangos said sane 
animals do not pass the foreign gene to their offspring; whether the gene will 
be passed to progeny depends on the time at which the microinjected DNA integrates 
into the chromosomal ENA. If chromosomal integration occurs at the pronuclear 
stage, the gene will be passed to offspring since all cells will contain the 
insert. If the gene does not integrate until several cell divisions have 
occurred, the gene will not be present in all the cells of the body and may not 
be contained in germ line tissue. The gene then will not be passed to offspring. 
Er. Scangos said cells in tissue culture have also been microinjected with 
foreign ENA. More than half of the cells surviving microinjection express the 
injected gene( s) . 
Dr. Scangos said when retroviruses are used to produce transgenic mice, generally 
only single copies of the viral DNA integrate and integration occurs at a 
single site within the chromosomal ENA. The insertion site is not under experi- 
mental control. Retroviral infection is usually initiated at later embryonic 
stages resulting in mosaic founder animals; outbreeding of the founder animal 
is therefore necessary to establish pure lines hemizygous for a single insertion 
site. Few embryos are lost when foreign ENA is introduced by retrovirus 
infection. An efficiency of approx imately 25% is possible w/ith viral vectors 
when they are used to infect pre implantation embryos. 
Er. Motulsky said use of microinjection techniques would not be ethically 
feasible in human sifcjects. The tremendous embryo losses associated with the 
microinjection technique would not be acceptable. In addition, it is not 
possible to determine whether the egg is nornal at the pronuclei stage. For 
most human recessive diseases, three fertilized eggs out of four would be 
normal for the trait in question. In order for all the fertilized eggs to 
possess the recessive trait, both nates would have to be homozygous for the 
deleterious trait. In humans, such a situation is highly unusual. 
Er. Mahoney suggested the embryo could be tested at the 8 cell stage when one 
cell could be sacrificed to test for the homozygous presence of the deleterious 
gene. If sane cells were modified at early embryonic stages, an individual 
mosaic for the trait could develop. In humans, mosaic individuals frequently 
do not display disease symptoms although they are carriers. 
Dr. Anderson said the microinjection technique is only successful at the pronuclear 
stage of development. The cell is designed to keep DNA out of the nuclei once 
the pronuclei fuse. 
[576] 
