beta Pix activates Rho family small GTPases, Rac1 and Cdc42 as a guanine nucleotide exchange factor. Although overexpression of beta Pix in cultured neurons indicates that beta Pix is involved in spine morphogenesis and synapse formation in vitro, the in vivo role of beta Pix in the neuron is not well understood. Recently, we generated beta Pix knockout mice that showed lethality at embryonic day 9.5. Here, we investigate the neuronal role of beta Pix using beta Pix heterozygous mice that are viable and fertile. beta Pix heterozygous mice show decreased expression levels of beta Pix proteins in various tissues including the brain. Cultured hippocampal neurons from beta Pix heterozygous mice show a decrease in neurite length and complexity as well as synaptic density. Both excitatory and inhibitory synapse densities are decreased in these neurons. Golgi-staining of hippocampal tissues from the brain of these mice show reduced dendritic complexity and spine density in the hippocampal neurons. Expression levels of NMDA- and AMPA-receptor subunits and Git1 protein in hippocampal tissues are also decreased in these mice. Behaviorally, beta Pix heterozygous mice exhibit impaired social interaction. Altogether, these results indicate that beta Pix is required for neurite morphogenesis and synapse formation, and the reduced expression of beta Pix proteins results in a defect in social behavior. (C) 2019 Elsevier Inc. All rights reserved.