In contrast, we focused on the activation of signal transduction via cell-cell interaction because the expression of various ligand molecules is induced downstream of NF-κB.64) When basal subtype breast cancer cell line HCC1937 with high NF-κB activation induced by ectopic expression of constitutively active IKKβ (HCC1937high) was co-cultured with the same breast cancer cells with unaltered NF-κB activation (HCC1937normal) for 12 days, the percentage of cancer stem cells in HCC1937normal became significantly higher than that in HCC1937normal without co-culture. The gene discussed is NFKB1; the disease is breast cancer.