1.In addition, healthy cells growing in log 
phase are critical for optimal transformation efficiency in gene targeting experiments.The criteria used in our laboratory to qualify an ES cell clone 
for making chimeras is that at least 50% of the chromosome spreads analyzed must be 
40 XY. In our experience, our DBA/1LacJ ES cells (12) meet or exceed that criterion 
Murine Embryonic Stem Cells 1
at least 86% of the time, whereas our 129 strain of ES cells meet or exceed the criteria 
45% of the time.Smithies and colleagues later demonstrated 
that ES cells, modified by gene targeting when reintroduced into blastocysts, could 
transmit the genetic modifications through the germline (7).ES cells have been reported for other mammalian 
species (i.e., hamster, rat, mink, pig, and cow), however, only murine ES cells have 
successfully transmitted the ES cell genome through the germline.ES cells are isolated from the inner cell mass (ICM) of the blastocyst stage embryo 
and, if maintained in optimal conditions, will continue to grow indefinitely in an 
undifferentiated diploid state.ES cells that are 
not cared for properly will spontaneously differentiate, even in the presence of feeder 
layers and leukemia inhibitory factor (LIF).Today, genetic modification 
of the murine genome by ES cell technology is a seminal approach to understanding the 
function of mammalian genes in vivo.(1-4).