In human patients, rare cases of disrupting one Htt allele did not develop abnormality (Ambrose et al., 1994), patients with homozygous mutant Htt alleles develop HD similar to heterozygous carriers (Dürr et al., 1999; Kremer et al., 1994; Squitieri et al., 2003; Wexler et al., 1987). This evidence concerns the gene HTT and Huntington disease.