Consistent with studies showing that small hippocampus, dentate gyrus, and cortex size are associated with anxiety and depressive disorders in mice and humans (Persson et al., 2014; Travis et al., 2015; Boldrini et al., 2013; Schmaal et al., 2017), Arid1b+/- mice have smaller dentate gyri (Figure 2B) and both Arid1b+/- and Arid1b-/- pups had reduced cortical thickness with reduced TBR1 marked neuronal cellularity (Figure 2—figure supplement 1A–D). This evidence concerns the gene ARID1B and depressive disorder.