SCN9A and breast cancer: Highly invasive MDA-MB-231 human breast cancer cells express mRNA for NaV1.5, NaV1.6 and NaV1.7 channels [16], but only NaV1.5 channels are functional at the plasma membrane and are giving rise to transient inward sodium currents (INa) under voltage-clamp procedures [13] (see Additional file 1: Material and methods).