SHH and Down syndrome: Even a single injection of an Shh agonist was shown to rescue neuronal numbers and behavioural deficits in an animal model of the syndrome.478 The gene encoding astrocyte-specific protein S100B is located in chromosome 21 and increased levels of S100B are characteristic for Down syndrome.479 Another cascade linking astrocytes to the pathophysiology of Down syndrome is an increased production of hydrogen sulphide, which is catalysed by cystathionine-β-synthase expressed, more or less exclusively, in astrocytes, and upregulated in a rat model of the disease.