GPC4 and Intellectual disability: Further support for essential HS functions in normal brain physiology comes from the observations that Ndst1 is a candidate gene for autosomal recessive intellectual disability in four unrelated families [64,65], that the Drosophila Ndst orthologue Sulfateless plays a role in normal social interactions and repetitive behavior in the fly [66], that Glypican 4 associates with hyperactivity and social interaction deficits in mice [67], and that a HS 6-O sulfotransferase gene variant associates with intellectual disability in humans [68].