The Herpes simplex virus thymidine kinase gene, including the polyadenylation signal, was ligated into the first exon of a mouse glial fibrillary acidic protein promoter cassette that contains all the exons, introns and promoter regulatory elements and includes 2.5 kb of 5' and 2 kb of 3' flanking genomic DNA. The GFAP protein is not expressed from the transgene because a small segment of exon one has been deleted. RT-PCR analysis showed co-expression of HSV-Tk and of Gfap mRNA (from the endogenous gene) in heart, lung, liver, spleen, adrenal, kidney and GI tract, as well as in brain and trigeminal ganglion; in neural tissues the co-expressing cells have the appearance of astroglia.