The human DNA contains the construction plan for every single component in the body. This construction plan consists of 3 billion letters. Sometimes, one single letter changes by chance, and this can change the construction plan and thus the component it encodes for (the protein).
Every single one of us has many genetic variants. Most of those variants do not cause any symptoms. However, some variants that cause symptoms are de novo variants.
SNAP25 mutations are almost always de novo. This means that the mutation arises anew spontaneously, and is not inherited from the mother or father. Therefore, the risk of having another affected child is quite low (less than 1%). SNAP25 Missense variations account for the greatest share of mutations, while nonsense and splice donor variants have also been reported.