This method can have answer key questions in the developmental biology field such as the study of gene expression patterns during embryonic development.The main advantages of this technique are that it is highly sensitive, easy and quick to perform and allows direct visualization of the staining in the whole mount tissue or at higher resolution after paraffin sectioning of the sample.Demonstrating the procedure will be Dr Maria Jose Blanco, Dr Anabel Learte, Dr Emma Munoz and Dr Miguel Marchena.All of them are senior post-doc from my lab.Begin by incubating glutaraldehyde-fixed transgenic embryos expressing galactosidase in freshly prepared X-gal rinse buffer for 10 minutes at room temperature.Use wild type litter-made embryos as negative controls for the enzymatic reaction.Next, replace the rinse buffer with X-gal staining solution at pH 8-9.Protect from light and incubate for 2 hours to overnight at 37