box domain that is found in the Drosophila paired 
(prd) and gooseberry (gsb) genes, and the Gli 
genes that have homology to the Drosophila cubi- 
tus interruptus ( Ci) gene. Analysis of gene expres- 
sion patterns and limited mutant analysis have indi- 
cated that these mammalian genes play important 
roles in pattern formation during gastrulation and 
organogenesis. In Drosophila, the wg, en, prd, gsb, 
and Ci genes have been shown to interact during 
segmentation in common genetic pathways. The ex- 
pression patterns of the mouse homologues of these 
genes have shown that they may also interact simi- 
larly in mammals. Thus it is possible that not only 
the genes but also the genetic pathways have been 
conserved through evolution. Alexandra L. Joyner, 
Ph.D. (Mount Sinai Hospital, Toronto) and her col- 
leagues have concentrated on determining the roles 
of the En genes in development and recently have 
begun to examine whether these genes interact with 
the Wnt, Pax, and Gli genes. 
Paul M. Lizardi, Ph.D. (National Autonomous Uni- 
versity of Mexico, Cuernavaca) and his colleagues 
are developing diagnostic assays based on RNA am- 
plification. An enzymatic reaction is used to join 
two RNA molecules that contain sequences comple- 
mentary to viral RNA. Joining occurs only if both 
probes are bound specifically to their target. The 
ligation reaction is mediated by a catalytic RNA 
known as a ribozyme. After joining, the resulting 
molecules are replicated by the enzyme Q-beta rep- 
licase, generating up to 100 million copies of the 
probe in 20 minutes. This novel method can be used 
for the rapid detection of infectious agents such as 
the AIDS virus. 
The aim of the research of James D. McGhee, 
Ph.D. (University of Calgary, Alberta) is to under- 
stand how and why specific genes are expressed in 
some tissues, but not others, during an animal's de- 
velopment. Genes are modified in a test tube and 
then injected back into the host organism, a small 
free-living roundworm called Caenorhabditis ele- 
gans, to see if the modified genes still work 
correctly. Results to date suggest that a gene is ac- 
tively turned on in the correct cells, at the same time 
that it is actively turned off in incorrect cells. These 
experiments are being extended to different round- 
worms to see how gene control mechanisms change 
during evolution. 
The laboratory of Tim Mosmann, Ph.D. (Univer- 
sity of Alberta, Edmonton) studies cytokines, small 
secreted proteins that mediate many of the functions 
of cells in the immune system. Two subsets of T 
lymphocytes, THl and TH2, have very different 
functions and cross-regulate each other via their 
unique cytokines. A new TH2-specific cytokine, 
P600, has been found to induce the growth of 
macrophage-like cells from bone marrow precur- 
sors. These cells are able to process and present anti- 
gens to T cells, but it is not yet apparent how the 
functions of these cells relate to the known unique 
properties of TH2 lymphocytes. 
Mechanisms involved in the virulence of Ent- 
amoeba histolytica are being investigated in the 
laboratory of Esther Orozco, Ph.D. (National Poly- 
technic Institute, Mexico City) by the identification 
of virulence-involved molecules and their expres- 
sion through the switching of trophozoites from the 
nonpathogenic to pathogenic states. The 112-kDa 
adhesin demonstrates protease activity and is altered 
in E. histolytica virulence-deficient mutants. This 
adhesin and other virulence-involved molecules 
were detected only in pathogenic trophozoites. Ge- 
nomic rearrangements, including gene amplifica- 
tion, correlate with the switching of trophozoites 
from nonpathogenic to pathogenic. The identifica- 
tion of a variable DNA fragment corroborated the 
high plasticity of the E. histolytica genome, which 
gives rise to genotypically and phenotypically dif- 
ferent individuals from a single cell. 
Many growth factors, and related hormones, regu- 
late cell function through cell surface receptors 
with protein-tyrosine kinase activity. To understand 
the means by which such tyrosine kinases function, 
it is important to identify their immediate targets 
within the cell. The laboratory of Tony Pawson, 
Ph.D. (Mount Sinai Hospital, Toronto) has identi- 
fied an element that appears to be a hallmark of tyro- 
sine kinase targets, the Src homology 2 (SH2) do- 
main. These domains of cytoplasmic signaling 
proteins bind to specific tyrosine-phosphorylated 
sites on activated growth factor receptors and 
thereby direct high-affinity interactions between re- 
ceptors and their targets. SH2 domains also mediate 
a network of protein-protein interactions in the cy- 
toplasm of cells stimulated with growth factors or 
transformed by oncogenic tyrosine kinases, which 
appear important to the control of signal transduc- 
tion and the induction of cell proliferation. 
The research in the laboratory of Lourival D. Pos- 
sani, Ph.D. (National Autonomous University of 
Mexico, Cuernavaca) is focused on the study of the 
structure and function relationship of scorpion tox- 
ins, which are the peptides responsible for the 
lethal effects of scorpion stings. Among the recent 
accomplishments are 1) the isolation of two un- 
known Na^ channel-blocking peptides from the 
venom of Centruroides noxius and the determina- 
tion of their primary structure and some of their 
immunological characteristics; 2) the chemical and 
functional characterization of two newly purified 
INTERNATIONAL RESEARCH SCHOLARS 497 
