However, TTX has good selectivity (at therapeutic doses) for some VGSC isoforms (e.g., Nav1.6 and Nav1.7) that are highly expressed in the nervous system, and in different studies such isoforms have been reported to be upregulated in chronic pain states (including neuropathic or cancer-related pain) [43,44,45,46]. Here, SCN9A is linked to cancer pain.