The human beta-globin gene and a functional viral thymidine kinase gene in developing mice.

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Abstract
Two foreign clones genes.sbd.1 encoding a tissue-specific protein and 1 encoding a constitutive enzyme.sbd.were introduced into mouse eggs by microinjection into a pronucleus shortly after fertilization. They were the adult human genomic .beta.-globin gene and the thymidine kinase (TK; ATP:thymidine 5''-phosphotransferase, EC 2.7.1.21) gene of herpes simplex virus (HSV), ligated in the pBR322 plasmid. Developing mice (33) were autopsied in late fetal life; all appeared normal. Blot hybridization tests revealed that the DNA of as many as 5 (15%) of the fetuses (from separate litters), and of their corresponding placentas, contained copies of the human .beta.-globin gene and of the HSV tk gene that was retained and replicated without significant loss or rearrangement. The estimated total numbers of copies per cell were 3-50 for the donor globin gene and 3-20 for the donor tk. In some of the fetuses, these totals included some copies of higher MW than that of the intact sequence; the additional segments may have arisen through changes such as deletions or duplications. The foreign genes in the 5 positive fetuses appear to be present in high MW DNA. Assays capable of distinguishing between foreign and native TK indicated that at least 1 of the fetuses with the HSV tk gene had some TK enzyme of the HSV type and, therefore, at least 1 gene copy that was being accurately transcribed and translated to produce a functional protein, despite the absence of selective pressure. Pure recombinant genes introduced into mice at the onset of their development can remain intact and be stably incorporated and even expressed. These experiments provide a practical basis for novel investigations of the developmental control of normal gene expression in vivo of the causes and possible cures of genetic diseases.