To elevate the expression frequency of transgenes in transgenic mice, the chicken lysozyme matrix attachment region (MAR) sequence was used by combining it with a transgene. The whey acidic protein (WAP) promoter/human lactoferrin (hLF) cDNA fusion transgene (pWL) was connected to the chicken lysozyme MAR sequence at its 5'-end (pMWL). While only two of three mice became transgenic from the pWL vector expressed hLF, all seven mice from the pMWL vector expressed the transgene in their lactating mammary glands. To evaluate the effect of lactogenic hormones on transgene expression, experiments with the primary culture of transgenic mammary explants were performed. It was revealed that the expression of transgenes was slightly increased by insulin plus dexamethasone or insulin plus prolactin treatment. However it was not increased by insulin, dexamethasone or prolactin (IDP) treatment alone. In contrast, the endogenous WAP gene was expressed only in the IDP treated group. These results demonstrate that MAR sequences are effective in improving the expression frequency of transgenes in transgenic mice although the developmental and hormonal regulations are not the same as those of the endogenous WAP gene.