HTT and Huntington disease: However, there is a lack of information as to whether the length of glutamine residues affects the binding of copper.122 Xiao et al. (2013) also reports a histidine residues being involved in binding, and also suggest that Cu2+ bind to the residue Met8.123 The same authors report that HD arises from a combinatory toxicity of PolyQ and Cu2+, that is, Cu2+ is actually required to cause HD.123 Interestingly enough, zebrafish that lack the huntingtin protein exhibit sizeable defects in iron utilization and development, meaning that huntingtin (PolyQ) may play a role in iron pathways.124