We studied a family where a mother passed on two different genetic variants at the same nucleotide in the GRIN2A gene to her children. Both variants disrupt the gene function. The son manifest epilepsy and intellectual disability, while the daughter and the mother exhibit language impairment and learning difficulties. The mother carries both variants, with one present in a small number of her cells. This complex genetic landscape complicates diagnosis since standard methods might miss minor variants. Specific technology is needed to detect such conditions, which might be more common than previously thought, severely affecting diagnostic accuracy and genetic counselling. (By Dr. Valentina Cetica, https://jmg.bmj.com/content/early/2024/07/31/jmg-2024-110101 )