As shown in Figure 5a, the ranking of the expression stability in these genes was (from most to least stable): GAPDH, YWHAZ, βACT, β2M, HPRT1, and GNB2L1. The M values of GAPDH were lower than 0.5, confirming that this gene is a stably expressed reference gene in human cortex and also validating the previously performed quantification of the BDNF mRNA in human HD samples versus controls [8]. Here, HPRT1 is linked to Huntington disease.