SLC9A9 and attention deficit-hyperactivity disorder: Given the known association of other endosomal NHEs (NHE6 and NHE9) with neurological diseases (e.g. NHE6 for Christianson syndrome and NHE9 for autism and ADHD) (Kondapalli et al., 2013; Ouyang et al., 2013; Pescosolido et al., 2014; Schwede et al., 2014; Prasad et al., 2017; Gao et al., 2019; Lee et al., 2021; Pescosolido et al., 2021) and potential functional redundancy of endosomal NHEs, it is possible that NHA2 also has an important, not yet understood, function in the brain or other organs that have not yet been studied in detail.