To identify cellular processes that may be regulated by ROBO1, we performed fluorescence-activated cell sorting (FACS) to purify populations of cells harvested from wild-type (WT) and Robo1tm1Matl/tm1Matl (herein referred to as Robo1−/− or KO) mature, virgin MGs: basal cells (Lin–CD24+CD29hi; BC), mature luminal cells (Lin–CD24loCD29+CD61–; ML), and luminal progenitor cells (Lin–CD24loCD29+CD61+; LP) (Harburg et al., 2014). Here, ROBO1 is linked to breast cancer.