In breast cancer, the α subunit gene (SCN5A) and nNav1.5 protein were found to be upregulated in the highly metastatic de novo resistant MDA-MB-231 cells, in contrast to the weakly metastatic MCF-7 (3,22), and are involved in the enhancement of extracellular matrix (ECM) degradation (23), in part through activation of acidic cysteine cathepsins B and S (24). This evidence concerns the gene SCN5A and breast cancer.