Notably, the tracing experiments by the knock-in of GFP-encoding gene into the endogenous SOX2 loci demonstrated that the SOX2-positive green cells isolated from heterogeneous tumor cell populations indeed express higher levels of stemness-related genes with characteristics of stem cells, as compared with SOX2-negative counterparts,149,153 confirming a causal role of SOX2 in cancer cell stemness. The gene discussed is SOX2; the disease is cancer.