TRIO variants that reduce TRIO GEF1 activity are associated with microcephaly (Barbosa et al., 2020; Bonnet et al., 2023; Bircher et al., 2022), so we assessed head and brain size in Trio variant mice (Figure 3A–E [males], Figure 3—figure supplement 1A–E [females]). The gene discussed is TRIO; the disease is microcephaly.