SLC9A6 and X-linked intellectual disability: A foundational study by Gilfillan et al. [8] identified four mutations in NHE6 (ΔV176-R201, H203fsX59, ΔE287-S288, R500X in the longest splice-variant of NHE6, NHE6v1, as illustrated in Fig. 1a) in patients affected by a severe form of X-linked intellectual disability originally identified in a multigenerational South African family [1] - since termed Christianson syndrome.