In order to determine the relationship between brain and CSF GAGs in the setting of IDS deficiency, we utilized a mouse model of MPS II (Ids KO) [32] crossed with a chimeric mouse knock-in (KI) model expressing the human TfR apical domain (Ids KO;TfRmu/huKI; described in References [31,33]), treated with increasing doses of a TfR-mediated brain penetrant form of IDS, referred to as ETV:IDS [31]. Here, IDS is linked to mucopolysaccharidosis type 2.