CUL4B and neoplasm: Unexpectedly, although most control MCF‐7 cells did not form tumors without E2 stimulation, tumors formed in mice injected with as few as 1 × 103 luciferase‐labeled MCF‐7‐CUL4B cells (6/8); CUL4B‐overexpressing cells showed a significant increase in CSC frequency and higher tumor‐formation capacity than those in the control group (Figure 7D, Figure S7C, Supporting Information 1).