Regulation of Human hst Expression by an Enhancer Element Residing in the Third Exon

The hst gene is exclusively expressed in undifferentiated embryonal carcinoma cell lines and at a limited stage of embryonal development. Two DNase I‐hypersensitive sites were mapped in the 3’region (approximately 3.5 and 4,5 kb downstream of the translations) initiation site) of the human hst gene, irrespective of the presence or absence of hst mRNA in the cells. A DNA fragment containing one of these DNase I‐hypersensitive sites (at around 3.5 kb relative to the translational initiation site) showed enhancer activity when tested by chloramphenicol acetyltransferase (CAT) assay. These results strongly suggest that an enhancer element(s) exists in the third exon of the hst and that the expression of the hst may be regulated by the presence or absence of a putative protein factor(s) which binds to the enhancer.