AQP4 is known to be involved in many brain diseases such as stroke and dementia (Manley et al., 2000; Aoki et al., 2003; Xu et al., 2015; Zeppenfeld et al., 2017; Abe et al., 2020), and AQP4 knockout (AQP4-ko) animals are often used to investigate the function of AQP4. The gene discussed is AQP4; the disease is brain disorder.