Cases of somatic mosaicism in many central nervous system (CNS) disorders have been reported, for example, somatic mutation in the presenilin-1 gene associated with Alzheimer’s disease (Beck et al., 2004), in SPG4/SPAST (spastic paraplegia4/spastin) causing spastic paraplegia (Depienne et al., 2007) or MECP2 (methyl CpG binding protein) resulting in Rett syndrome (Topçu et al., 2002). The gene discussed is SPAST; the disease is Rett syndrome.