Retroviral Replication 
can selectively integrate the viral DNA into pre- 
determined sites in the target DNA. 
The ultimate goal of our work on integration is 
to understand in molecular detail how the pro- 
teins of the integration machinery recognize the 
viral DNA, assemble into an active complex, rec- 
ognize the target DNA, and finally catalyze the 
DNA breakage and joining reactions that lead to 
integration of the provirus. It is hoped that this 
understanding will lead to the development of 
new agents for inhibiting the replication of patho- 
genic retroviruses and to improved systems for 
the therapeutic introduction of genes into mam- 
malian cells. 
It has been recognized for many years that es- 
tablishment of a retroviral provirus proceeds 
much more readily in actively dividing cells than 
in their resting counterparts. However, the basis 
for this phenomenon remains obscure. To bring 
this observation into clearer focus, we have in- 
vestigated the dependence of specific steps in the 
life cycle of the murine leukemia virus on the 
host cell's stage in its own division cycle. Using 
drugs or a mutation in the cell-cycle control gene 
cdc2 to regulate the cell cycle of the host cells, 
we have found that integration in vivo depends 
on mitosis. Yet synthesis of viral DNA and accu- 
mulation of integration-competent intermediates 
occur normally in cells blocked from entering 
mitosis. The intermediates accumulate in the cy- 
toplasm and remain stable for hours, awaiting mi- 
tosis. Current investigations are aimed at under- 
standing why mitosis is required for integration. 
Preliminary evidence suggests that disassembly 
of the nuclear envelope at mitosis may provide a 
route of entry for viral replication intermediates. 
Understanding how cellular functions can deter- 
mine the fate of an infecting retrovirus may lead 
to new approaches to antiviral therapy and to im- 
provements in the use of retroviruses as vectors 
for gene therapy. 
Our work on retroviral integration is supported 
by a grant from the National Institutes of Health. 
New Methods for Linkage Mapping 
in Complex Genomes 
A major impediment to defining and character- 
izing the genes that influence complex human 
traits has been the difficulty of collecting suitable 
large families in which the trait segregates. Such 
families are generally needed to find genes by 
conventional linkage mapping. The same genes 
could in principle be mapped more easily by an 
alternative strategy that involves collecting and 
analyzing pairs of relatives that share a trait of 
interest. However, linkage mapping with small 
sets of relatives generally requires analysis of a 
large number of closely spaced and highly poly- 
morphic genetic markers, which makes this strat- 
egy impractical with current technology. 
We are developing a new set of genetic tools 
that will allow widespread application of these 
highly efficient linkage mapping methods. Ex- 
periments are in progress to test these new meth- 
ods in model systems. In parallel with our efforts 
to develop new biochemical methods, we are 
working to find optimal statistical methods, using 
high-resolution maps of genetic identity between 
pairs of relatives. Our aim is to apply this technol- 
ogy to map genes for complex human traits. 
This work is supported by a grant from the Na- 
tional Institutes of Health. 
54 
