MACROH2A1 and Huntington disease: The H2A histone family, member Y (H2afy) gene, which encodes the macroH2a1 histone variant, has been found to be a clinically relevant HD biomarker that is overexpressed in the blood (mRNA) and brains (protein) of HD patients, as well as in the brains of two HD mouse models (Hu et al., 2011).