Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are incurable neurodegenerative diseases with shared pathology and genetics .  We know how common ALS and FTD are, how often they run in families and how often specific genes are implicated.  We used published literature and population-level data about genomic variation to calculate how common disease-causing variants are in four common ALS genes.  We found more disease-causing variants in the population than expected, meaning many people carry such alterations without ever developing neurodegeneration.  The reason for this remains unknown but it suggests existence of as-yet unidentified protective factors in the population. (https://jmg.bmj.com/content/early/2023/12/16/jmg-2023-109580 )