The c-fos-tTA transgene was designed with the promoter region from the mouse FBJ osteosarcoma oncogene (Fos or c-fos; at least 611 bp of 5' untranscribed sequence that encompasses the known mouse c-fos regulatory elements) placed upstream of the tetracycline-regulated transactivator gene (tTA or "Tet-Off" ; flanked by an artificial intron and splice sites at the 5' end and by a polyadenylation signal from SV40 at the 3' end). Specifically, the tTA was inserted into the AccI site of the first exon of the mouse c-fos gene, (J:151755)