Additional Supporting Information may be found in the online version of this article.

DVDY_22539_sm_suppfig1.tif1306KSupporting Figure 1 Prom1−/− tissue responds normally to bFGF Organoids from Prom1+/+ and Prom1−/− mammary tissue grown in Matrigel for 7 days, with or without bFGF supplementation. Scale bar = 100 μm.
DVDY_22539_sm_suppfig2.tif1282KSupporting Figure 2 Prom1 loss does not cause abnormal cellular composition or polarity in mammary ducts. Tissue sections from Prom1+/+ and Prom1−/− mice labeled with antibodies against (a) cytokeratin 18 (K18, red) and cytokeratin 14 (K14, green), and (b) ZO1 (red) and K14 (green), showing normal distribution in the mammary ducts of both genotypes. Scale bars = 50 μm.
DVDY_22539_sm_suppfig3.tif311KSupporting Figure 3 Prom1 expression decreases during pregnancy. Immunofluorescence of Prom1 (red) expression in virgin mammary tissue and throughout pregnancy. Scale bars = 50 μm.
DVDY_22539_sm_suppfig4.tif879KSupporting Figure 4 Prom1 loss is associated with increased luminal cell proliferation in vitro. a: Individual colonies established in 2D culture, labeled for K18 (red) and smooth muscle actin (SMA, green). Scale bars = 100 μm. b: Western blot analysis of ERK activation (phosphorylation) in mammary tissue from 4–5-week-old Prom1+/+ and Prom1−/− mice, with a β-actin loading control. c: RT-PCR analysis of cyclin D1 (Ccnd1) cDNA levels in mammary tissue from 4–5-week-old Prom1+/+ and Prom1−/− mice, with a Gapdh control. d: Quantification of the area of individual colonies from Prom1+/+ and Prom1−/− cells collected from virgin (n=2) and parous (n=4) mice. *P < 0.05, **P < 0.01. Scale bars = SD.
DVDY_22539_sm_suppfig5.tif461KSupporting Figure 5 Prom1 loss does not affect cilia distribution. Immunofluorescence labeling for cilia (acetylated α-tubulin, red) in (a) ovarian follicles and (b) mammary ducts, from Prom1+/+ and Prom1−/− mice. Scale bars = (a) 25 μm, (b) 50 μm. O, oocyte; fc, follicle cells.

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