Loss-of-function mutations in the catenin beta 1 (CTNNB1) gene lead to β-catenin deficiency, which is strongly associated with the CTNNB1 syndrome, a rare, monogenic, neurodevelopmental disorder marked by an array of motor and cognitive impairments with an estimated prevalence of around 2.6–3.2 in 100,000 live births.1 This evidence concerns the gene CTNNB1 and Down syndrome.