to the second-stage problem of mapping multi- 
mega-base pair contigs in sufficient detail to sup- 
pon large-scale DNA sequencing. This application 
requires the mapping of enormous numbers of sites. 
Even a 1 -Mbp region, mapped at a resolution of 1 00 
bp, requires the mapping of 10,000 sites. Further- 
more, the need to solve such problems repeatedly 
requires an automated strategy. Emphasis is pres- 
ently on the computational problems posed by auto- 
mated high-resolution mapping projects. Tradition- 
ally computers have been used as little more than 
bookkeeping tools in physical mapping projects; 
most substantive decisions have been made by a hu- 
man analyst. However, the scale of the mapping 
problems posed by the human genome requires that 
interactive human involvement in the data analysis 
be minimized. 
Functional Studies 
YACs also provide a path toward the functional 
analysis of large segments of the chromosomes of 
higher organisms. The experimental approach in- 
volves cloning the segments into yeast and then 
transferring the YACs into mammalian cells where 
their genetic effects can be evaluated. Test systems 
involve the human HPRT (hypoxanthine phosphori- 
bosyltransferase) and GART (phosphoribosylglycin- 
amide formyltransferase) genes, both of which en- 
code enzymes required for the de novo synthesis of 
purines in mammalian cells. Both human genes have 
been cloned in YACs and transferred to mutant ro- 
dent cells that lack HPRT and GART activities. The 
cloned genes have proved to be fully functional in 
the rodent cells to which they were transferred. 
Past successes in transferring YACs to mammalian 
cells have largely relied on fusing mammalian cells 
with yeast cells whose cell walls have been re- 
moved. Although effective in some cases, cell fusion 
fails with many mammalian cells. A more robust 
procedure is microinjection of the YAC DNA. The 
fragility of large DNA molecules has required the 
development of specialized handling procedures to 
allow microinjection of intact YACs. During the 
past year, major improvements in such procedures 
have allowed intact DNA segments as long as 500 
kbp to be transferred to mammalian cells by 
microinjection. 
Dr. Olson is also Professor of Genetics at Wash- 
ington University School of Medicine, St. Louis. 
Effective September 1, 1992, he will assume the 
position of Professor of Molecular Biotechnology 
at the University of Washington, Seattle. 
Books and Chapters of Books 
Olson, M.V. 1991 . Genome structure and organiza- 
tion in Saccharomyces cerevisiae. In The Molecu- 
lar and Cellular Biology of the Yeast Saccharo- 
myces: Genome Dynamics, Protein Synthesis, 
and Energetics (Broach, J.R., Pringle, J., and 
Jones, E.W., Eds.). Cold Spring Harbor, NY: Cold 
Spring Harbor, pp 1-39. 
Articles 
Green, E.D., Mohr, R.M., Idol, J. R , Jones, M., Buck- 
ingham, J.M., Deaven, L.L., Moyzis, R.K., and 
Olson, M.V. 1 99 1 ■ Systematic generation of 
sequence-tagged sites for physical mapping of hu- 
man chromosomes: application to the mapping of 
human chromosome 7 using yeast artificial chro- 
mosomes. Genomics 11:548-564. 
Green, E.D., Riethman, H.C., Dutchik, J.E., and Ol- 
son, M.V. 1991. Detection and characterization 
of chimeric yeast artificial-chromosome clones. 
Genomics 11:658-669. 
Kwok, P.-Y., Gremaud, M.F., Nickerson, D.A., Hood, 
L., and Olson, M.V. 1992. Automatable screening 
of yeast artificial-chromosome libraries based on 
the oligonucleotide-ligation assay. Genomics 
13:935-941. 
GENETIC HEMATOLOGY 
Stuart H. Orkin, M.D., Investigator 
Hematopoietic cells develop from pluripotent 
stem cells in the bone marrow. During their differ- 
entiation, they progressively commit to specific lin- 
eages, such as red, white, and lymphoid cells, and 
the cells of each lineage synthesize characteristic 
gene products. The efforts of this laboratory are di- 
rected toward an understanding of the molecular 
basis of cell commitment and of the mechanisms 
that regulate coordinated expression of genes in de- 
veloping blood cells. Findings derived from these 
studies will provide further understanding of cellu- 
lar development in vertebrates and may bear on the 
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