NRG1 and amyotrophic lateral sclerosis: The reason for NRG1 upregulation in ALS is unclear; we postulate that upregulation of the NRG1 ligand may occur as a feedback loop in response to attenuated ERBB4/HER4/ALS19 signaling due to loss-of-function mutation or dampened expression for other reasons and may in turn upregulate other ERBB receptors [9, 11].