ARHGEF1 and Neurodevelopmental delay: NDD-associated TRIO variants affect in vitro GEF1 and GEF2 activities, cell morphology, inhibitory neuron maturation, axon guidance, and synapse function (Katrancha et al., 2019; Sun et al., 2021; Tian et al., 2021; Bircher et al., 2022; Barbosa et al., 2020; Bonnet et al., 2023); however, the impact of heterozygosity for distinct TRIO variants in vivo, as found in individuals with NDDs, has been unexplored.