SCN9A and hereditary sensory and autonomic neuropathy: Although endogenous opioids have been suggested to be upregulated in the absence of functional NaV1.7 and to contribute to the CIP phenotype (Minett et al., 2015), we did not see upregulation of PENK mRNA in NaV1.7 KO iPSC nociceptors or meaningful expression to begin with in healthy control neurons.