y gland lysates prepared from 10-week old MMTV-LRP6 virgin mice was performed. An antibody to detect both endogenous mouse LRP6 as well as the transgenic human LRP6 was used. We found that mammary glands from the transgenic MMTV-LRP6 Founder 4 and 1 displayed 2.6 fold and 1.7 fold greater LRP6 expression levels, respectively, compared to mammary glands from WT littermate controls ([Figure 1c](#F1){ref-type="fig"}). Furthermore, immunohistochemical staining confirmed that LRP6 expression in transgenic MMTV-LRP6 virgin glands was higher than WT virgin glands ([Figures 2a, b](#F2){ref-type="fig"}).

Mammary Gland Hyperplasia in MMTV-LRP6 Transgenic Mice {#S4}
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Whole mount preparations are a well-established method to identify early premalignant lesions of the mammary epithelium ([@R7]). Thus, we performed whole-mount staining of virgin glands to examine the ductal structure of mammary glands in MMTV-LRP6 transgenic mice and WT littermate controls. As shown in [Figure 3a](#F3){ref-type="fig"}, examination of a mammary gland taken from a wild-type littermate control female mouse revealed a branching ductal structure typical of a virgin female. In contrast, inspection of the MMTV-LRP6 mammary gland revealed an unusual number of secondary and tertiary branches and small, spiculated side buds. Quantification of terminal end buds (TEBs) from mammary glands revealed that TEBs from MMTV-LRP6 virgin mice (Founder 4) at 14 weeks (n=4) and 21 weeks (n=4) of age are 3.3 and 6 folds higher, respectively, than those from WT littermate control glands ([Figure 3b](#F3){ref-type="fig"}). Both the founder 1 and 4 lines displayed similar mammary epithelial abnormalities with the founder 4 line having more significant hyperplasia. Furthermore, histological sections of virgin glands showed more individual ducts lined with cuboidal epithelium in transgenic mammary glands than WT littermate control glands ([Figures 2c, d](#F2){ref-type="fig"}). These findings indicate that overexpress