trus (*n* = 10) or estrus (*n* = 9). Data represent mean ± SD, Student *t*-test, two-tailed. **h** Dot plot showing *Adamts18* mRNA levels normalized to *Krt5* in mammary glands from mice shown in **g**. Data represent mean ± SD, Student *t*-test, two-tailed. **i** Dot plot showing *Adamts18* mRNA normalized to *Hprt* in 6 contralateral mammary glands transplanted with *WT.EGFP*^*+*^ or *PR*^*−/**−*^*.EGFP*^*+*^ epithelium. **j** Bar graph showing relative transcript expression of different Wnt signaling components normalized to *Hprt* in contralateral glands of 8 mice transplanted with *WT.EGFP*^*+*^ and *PR*^*−/**−*^*.EGFP*^*+*^ epithelia. Each data point represents one gland, mean ± SD, paired Student *t*-test, two-tailed. **k** Dot plots showing relative transcript levels of *Wnt1*, *Axin2* and *Adamts18* normalized to *Hprt* in mammary glands from 5 *WT* and 3 *MMTV-Wnt1* virgin mice. Data represent mean ± SD, Student *t*-test, two-tailed. **l** Representative micrographs of *Adamts18* mRNA localization, (red) dots, in mammary glands from 3 *WT* and 3 *MMTV-Wnt1* females, α-Sma (green) and DAPI (blue); arrows show myoepithelial cells. Scale bar, 50 μm. **m** Dot plots showing mRNA levels of *Wnt4* and *Adamts18* normalized to *Hprt* in contralateral glands of 3 mice transplanted with *WT.EGFP*^*+*^ and *Wnt4*^*−/**−*^*.EGFP*^*+*^ epithelia harvested at 8.5-day of pregnancy. \**p* \< 0.05; \*\**p* \< 0.01; \*\*\**p* \< 0.001; \*\*\*\**p* \< 0.0001.

Analysis of *Adamts18* transcript levels at different stages of mammary gland development revealed low prepubertal expression that increased 2.7, 7- and 8.6-fold in 4-, 6- and 8-week-old females, respectively; expression rose further during pregnancy with a peak at mid-pregnancy day10.5/12.5 (Fig. [1b](#Fig1){ref-type="fig"}). RNAscope in situ hybridization for *Adamts18* transcripts combined with immunofluorescence (IF) for the myoepithelial marker α-smooth muscle actin (Sma) confirmed myoepithelium-sp