It remains to be determined whether these proteins as well as other factors controlling the dynamics of transcription elongation, including positive transcription-elongation factor-b (P-TEFb), negative elongating factor (NELF) or DRB sensitivity-inducing factor (DSIF), a factor composed of SPT4 and SPT5 [61]–[63], contribute to tissue-selective CAG instability in HD. The gene discussed is SUPT4H1; the disease is Huntington disease.