ABSTRACT
Only a small fraction of the gene pool of cells is expressed at any given time. The transcription of the bulk of the genetic material is silenced either by negative regulators (Johnson, 1995) or by the lack of transcriptional activators (Tjian and Maniatis, 1994; Tjian, 1995; McKnight, 1996). Nuclear transplantation (Gurdon, 1962; Gurdon et al., 1975) and heterokaryon studies (Davidson, 1972; Blau et al., 1983; Blau, 1992) provided the first evidence that silent genes can be activated by varying the gene dosage.
