In these xenografts of neuroblastoma (SKN-BE-2), sarcoma (HOS and RH4), small cell lung cancer (H-146), colon cancer (LOVO), breast cancer (MCF-7), and oral squamous cell cancer (SCC-25), we found that migratory side population cells (SPm) enriched in ABCG2+ CSCs having high tumorigenic activity reside in the hypoxic TME niche and exhibit TS phenotype (5–7, 11). The gene discussed is ABCG2; the disease is breast cancer.