"In situ" hybridization is a widely used technique for studying gene expression. Here, we describe two experiments addressed to postgraduate genetics students in which the effect of transcription factors on gene expression is analyzed in "Drosophila embryos of different genotypes by whole-mount in situ hybridization. In one of the experiments, students analyzed the repressive effect of Snail over "rhomboid" expression using reporter lines containing different constructs of the "rhomboid" neuroectodermal enhancer fused to the "lacZ" gene. In the second experiment, the epistatic relationship between the "cabut" and "decapentaplegic" genes was analyzed. These simple experiments allowed students to (1) understand the role of transcription factors and "cis"-regulatory elements over gene expression regulation and (2) practice a widespread laboratory technique, "in situ" hybridization with nonradioactive labeled probes, to detect gene expression patterns. These experiments required 12 hr and were organized into four daily sessions that included the discussion of the results with students. Examples of the results obtained and their relevance are shown and discussed herein. The methods described in these laboratory exercises can be easily adapted to model organisms other than "Drosophila."