Considering that 4T1 are p53-deficient breast cancer cells (33), a high NO level, promoted by the NF-κB signalling pathway in the tumour of untreated 4T1-challenged mice, may contribute positively to the tumour progression and metastasis by inducing expression of c-myc, VEGF and MMP-9 genes. The gene discussed is MMP9; the disease is breast cancer.