LGR5 and breast cancer: Groups of contiguous labeled cells touching each other (termed “doublets” here) containing BCs and LCs were observed in both Lgr5 and Lgr6 only following higher doses of TAM that lead to a nonlinear increase in the chimerism (Fig. 1C,F,J,M; Supplemental Fig. S1D), potentially suggesting that the BC lineage may contain a rare subpopulation of bipotent SCs that is only labeled at higher doses of TAM (Supplemental Fig. S1D; Supplemental Table S7).