The affected individual was clinically diagnosed with atypical RTT, but genetic testing showed no pathogenic variants in MECP2, CDKL5, or FOXG1. Singleton whole genome sequencing was conducted, which identified a heterozygous stop–gain variant [NM_001170629.2: c.5017C>T, p.(Arg1673⁣∗)], in the chromodomain-helicase-DNA-binding protein 8 (CHD8) gene. This evidence concerns the gene CDKL5 and Rett syndrome.