Protooncogenes expressed in murine embryonal carcinoma (EC) cells or their differentiated daughter cells include more or less ubiquitously expressed protooncogenes such as c-myc, c-K-ras, and c-abl, as well as c-onc genes with a very restricted expression pattern. Examples of the latter are N-myc, c-mos, and int-2. These c-onc genes are transcriptionally active in EC cells, as well as in germ cells and/or early embryonic cells. When EC cells are induced to differentiate some protooncogenes or oncogene-related products undergo changes in expression. Thus, EC cell differentiation has been associated with increased expression of c-src, c-fos, int-1, int-2, and the epidermal growth factor (EGF) receptor, whereas decreased expression has been observed for c-mos, c-K-ras, c-myc, N-myc, and platelet-derived growth factor. The relationships between these changes in expression and EC cell differentiation are not understood. They may be important for the differentiation process or for expression of a differentiated phenotype. They may, however, also be secondary events with no functional significance to EC cell differentiation.