 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.imagesfigure 2.figure 2.figure 4.figure 4.figure 5.figure 5 . 
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