Variable expression of human transgenes in SHN mice.

The mouse mammary tumour virus/human transforming growth factor alpha (MMTV/hTGF alpha) gene or the mouse whey acidic protein/human growth hormone (mWAP/hGH) gene was introduced to a high mammary tumour strain of SHN virgin females by mating with males bearing each gene. We maintained transgenic mice by backcrossing males with the hTGF alpha transgene or high serum hGH levels (> 50 ng/ml) to SHN virgins in the subsequent generations. Expression of the transgenes was examined at each generation. At the first and the second generations of backcrossing (N1 and N2), females showed hTGF alpha mRNA in the mammary glands associated with a marked outgrowth of the glands. Both females and males had high hGH levels in the circulation, but these characteristics disappeared completely after the third generation (N3). All findings indicate that SHN mice are specific in the resistance to the expression of some human transgenes.