(Liu et al. 2017. Article in Chinese) In the past several years, careful and sensitive DNA sequencing techniques have revealed that a small but significant percentage of parents of patients with DS who initially test negative for the child’s mutation are actually mosaic for the mutation. That is, instead of the mutation being present in all of their body’s cells (which would result in a positive blood test), it is only present in a portion of them, likely because the mutation occurred some time after the first cell division after fertilization, so that only those cells descended from the original mutated cell carry the mutation. This study looked at 575 Chinese families with a DS child and found 30 cases of parental mosaicism (or 5%). 13 of those 30 parents had no symptoms, 10 had a history of febrile seizures, 5 had epilepsy, 1 had febrile seizures plus, and 1 had an afebrile seizure. Four of the 30 mosaic families had two children with DS. Two families received positive prenatal SCN1A results after a sibling was diagnosed with DS, and one pregnancy was terminated.

Liu AJ, Yang XX, Xu XJ, Wu QX, Tian XJ, Yang XL, Wu XR, Wei LP, Zhang YH. [Study on mosaicism of SCN1A gene mutation in parents of children with Dravet syndrome]. Zhonghua Er Ke Za Zhi. 2017 Nov 2;55(11):818-823. doi: 10.3760/cma.j.issn.0578-1310.2017.11.005. Chinese. PubMed PMID: 29141311.