To understand the transcriptional regulation of IRBP, there was a need for a sensitive assay for IRBP mRNA. This chapter describes a ribonuclease protection assay that can be used to determine the emergence and steady state levels of IRBP mRNA during retinal development. This chapter shows the way by which the RPA has been used to assess endogenous IRBP gene expression in two families of transgenic mice. As a reference to IRBP, the analysis is also extended to include the mouse opsin gene and the IRBP-chloramphenicol acetyltransferase (CAT) transgene in the study. These transgenic mice offer a unique opportunity to compare the expression of two genes within the same mouse: (1) an endogenous gene directed by its own promoter and (2) a transgene directed by an evolutionally conserved promoter. As a first step toward understanding the way by which expression of the IRBP gene is regulated by the differentiation state of the photoreceptor cells, the PRA is used to quantitatively determine and compare the steady state levels of mRNAs for the IRBP-CAT transgene and the endogenous IRBP gene during development.