*2.1. Reagents*

Antibodies recognizing NANOG (D73G4), SOX2 (D6D9), KLF4 (D1F2), HO1 (D60G11), Vimentin (D21H3) and Keap1 (D6B12) were obtained from Cell Signaling Technology (Danvers, MA, USA). E-cadherin (610404) was obtained from BD Biosciences (San Jose, CA, USA). Nrf2 (ab-89443), SLUG (ab-27568), SNAIL (ab-53519) and Bach1 (ab-115210) were purchased from Abcam, (Burlingame, CA, USA) and NQO1 (sc-32793), BAX (sc-7480), BCL2 (sc-7382) were obtained from Santa Cruz Biotechnology (Dallas, TX, USA). Nrf2-targeting shRNA lentiviral particles (sc-37030-v) were obtained from Santa Cruz Biotechnology (CA, USA). Bovine serum albumin (BSA), fibroblast growth factor (FGF) and epidermal growth factor (EGF) were purchased from Sigma-Aldrich (St. Louis, MO, USA). ALDEFLUOR Kit (01700) was obtained from STEM CELL technologies (Vancouver, Canada). Nrf2 Transcription Factor Assay Kit (Colorimetric) (ab207223) was obtained from Abcam. Annexin-V-FITC (556419) was obtained from BD Biosciences (San Jose, CA, USA).

#### *2.2. Cell Culture and Irradiation*

MCF-7 and MDA-MB-231 cells were obtained from the American Type Culture Collection (ATCC) and maintained in DMEM supplemented with 10% FBS (Gibco, MD, USA). Cells were irradiated at room temperature with a <sup>60</sup>Co-γ rays laboratory irradiator (Gamma Chamber-5000, BRIT, Mumbai) at a dose rate of 2.163 Gy/min for the time required to obtain the prescribed dose. For fractionated doses of radiation, cells were irradiated with 2Gy for three consecutive days, and for an acute dose of 6Gy, cells were irradiated once on

the last day of fractionated irradiation. Corresponding controls were mock irradiated [13]. Control and irradiated cells were further incubated for 24 h postirradiation.

### *2.3. Mammosphere Formation*

After irradiation, mammospheres were formed using single-cell suspensions in an ultralow attachment 6-well plate at a density of 2 <sup>×</sup> <sup>10</sup><sup>4</sup> cells/well in specific media for mammosphere culture containing DMEM and Nutrient Mixture F-12 medium supplemented with 20 ng/mL EGF, B27 (1:50, Life Technologies, MA, USA), 20 ng/mL FGF (Sigma-Aldrich, St. Louis, MO, USA) and penicillin/streptomycin (Invitrogen) for 4–5 days. The floating aggregates with a >50 µm diameter were selected as mammospheres, manually counted and dissociated by incubation with 1:5 of 0.25% trypsin/EDTA. The mammosphere-forming efficiency (MFE) was calculated using the following equation: MFE = No. of spheres formed/No. of cells seeded × platting efficiency.
