Efficient BLG-Cre mediated gene deletion in the mammary gland

Using the phage P1-derived Cre/loxP recombination system, we have developed a strategy for efficient mammary tissue specific inactivation of flexed ge nes. Transgenic mice were generated which express Cre DNA-recombinase under the control of the mammary gland specific promoter of the ovine beta-lacto globulin (BLG) gene. To test the specificity of Cre mediated recombination, we crossed these mice to animals harbouring a flexed DNA ligase I allele. We show that the BLG-Cre construct specifies mammary specific gene deletion , and furthermore that it is temporally regulated, predominantly occurring during lactation. We fully characterised the extent of gene deletion in one line (line 74). In this strain the virgin gland is characterised by low le vels (7%) of Cre mediated deletion, whereas 70-80% of cells within the lact ating mammary gland have undergone recombination. Immunohistochemistry and indirect in situ PCR were used respectively to demonstrate that both Cre pr otein and Cre activity were evenly distributed throughout the population of secretory epithelial cells. The level of background recombination in non-m ammary tissues was found to be less than or equal to 1.1%, irrespective of mammary gland developmental status. Crossing the transgenic BLG-Cre strain described here to mice harbouring other flexed alleles will facilitate the functional analysis of those genes during differentiation and development o f the mammary gland.