Publisher Summary Cyclic AMP-dependent protein kinase (cAPK) is the principal target of cAMP in most animal cells and, thus, mediates cellular responses to hormones that affect levels of cAMP. In mice, the cAPK family of isozymes includes four regulatory subunits, namely, RIα, RIβ, RIIα, and RIIβ, and two catalytic subunits called Cα and Cβ. The early passage embryonic stem (ES) cells contribute to the germ line more readily than late passage cells, because of the accumulation of chromosomal abnormalities that occurs in culture. This chapter describes the establishment of the cell lines and their frequency of contribution to the germ line of chimeric animals. In an experiment described in the chapter, four pluripotent ES cell lines were isolated and named REK1, REK2, REK3, and REK4. Each of the four ES cell lines contained some cells with non-ES morphology, ranging from approximately 5% to 40% of the total cells. While ES cells grew as indistinguishable members of a smooth colony, these contaminating non-ES cells were individually distinct, growing in a monolayer with a cobblestone-like appearance. The establishment of mouse strains carrying mutations in the cAPK genes allows the study of the function of the various subunits in multiple tissue and cell types. RIβ and RIIβ are both expressed at high levels in the nervous system and therefore, mutant mice have altered neuronal biochemistry.