HEXA and Cerebral visual impairment: Torres et al. (2010) showed that in Jacob sheep, TSD clinical manifestations are closest to the pathological features in humans. Sheep with TSD also suffered from ataxia, proprioceptive defects and cortical blindness (Porter et al., 2011). Genetic studies showed that HexA activity deficiency in these sheep is associated with a single nucleotide substitution in exon 11 of the HEXA gene, which leads to glycine-to-arginine substitution (Torres et al., 2010).