To define a gene expression mechanism, it is often advantageous to use a reporter gene and transgenic mouse. The lacZ reporter gene is particularly useful for studies of the cis-regulatory element for tissue-specific expression in transgenic mice because of the ease of the enzyme assay and visualization on sections. In this report, we describe our method for examining the cis-regulatory element in transgenic mice, including choice of the lacZ gene, generation of transgenic mice, and analysis of beta-galactosidase activity.