Prox1is a negative regulator of mammary epithelial progenitor cellsin vitro.(A) qRT-PCR detection of Prox1 transcript abundance in MaSC/basal cells following transduction with retroviruses expressing shControl or shRNAs targeting Prox1. Data are shown as mean ± S.E.M. Prox1 expression normalized to Ywhaz1 expression relative to shControl (n = 2). (B) Transduced cells (500) were plated on an i3T3 feeder layer and cultured for 6 days to allow the formation of colonies. Left panel: representative images of observed colonies. Right panel: histogram showing the number of colonies derived from cells transduced with shControl- and shProx1-retroviruses. Data are shown as mean ± S.D for three independent experiments. (C) Haematoxylin and eosin staining and anti-P63 (myoepithelial) and anti-K8/K18 (luminal) immunohistochemical staining of outgrowths derived from cells transduced with a control versus shProx1 retrovirus. Scale bars, 50 μm.