Lac-Z Staining Sensitivity?

[veronica narvaez]

Hi All!
I have a problem detecting lac z as "blue staining" when using X-gal 
intransgenic mice (staining of whole embryos). By RT-PCR I know that
my lac z gene is being expressed, but I can not detect anything blue 
in the embryo. This could be due to too low levels of expression. 
Would any of you know how many molecules of Lac Z a cell
actually needs to produce in order to be detectable as a "blue" cell?
Thank you very much!
Veronica

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Veronica Narvaez
National Institute for Medical Research
v-narvae at nimr.mrc.ac.uk