Point mutations or rearrangements involving the PCDH19 gene located on Xq22.1 cause epilepsy and mental retardation mainly in females. However, affected males with mosaic PCDH19 mutations have also been reported. It is hypothesized that cellular interference mechanism is the main pathogenic mechanism.

In our 42 PCDH19-related epilepsy families, five mosaic individuals, four males and one female, were identified in total. The observation of asymptomatic mosaic fathers implied that the frequency of paternal mosaicism might be underestimated. The importance of our study is to provide significant knowledge about somatic mosaicism phenomenon in males and females, guide genetic counselling, and subsequent functional studies on mosaic males with different penetrance. （By Dr. Aijie Liu, https://jmg.bmj.com/content/early/2018/10/03/jmedgenet-2017-105235 ）