For SCN1A-related disorders, 10% of parents with (likely) pathogenic variants remain asymptomatic while offspring inheriting variants from affected parents have less than a 100% chance of developing seizures both due to incomplete penetrance and variable expressivity (https://www.ncbi.nlm.nih.gov/books/NBK1318/, SCN1A seizure disorders, Gene Reviews, last assessed in 14 May 2024). Here, SCN1A is linked to epilepsy.