ABCG2 and cancer: The mRNA species that were up‐ or downregulated in 4T1 cells in response to radiation are therefore shown in Fig. S3A. Genes upregulated in irradiated 4T1 cells included Aldh1a1 (Nozaki et al., 2017) and inducible nitric oxide synthase Nos2 (Kim et al., 2013) known to be increased in TIC, Pecam‐1, Lin28A, and Ptprc/CD45 which participate in cancer and embryonic stemness (Lin et al., 2014; Shyh‐Chang and Daley, 2013; Williamson et al., 2013), luminal progenitor marker c‐Kit (Visvader and Stingl, 2014), and ABCG2, a protein involved in SC chemoresistence (An and Ongkeko, 2009).