HTT and Huntington disease: In addition, some studies on animal models have indirectly demonstrated impairments in developmental myelination in HD: these have shown that the expression of a mutant huntingtin transgene in cells and in R6/2 transgenic mice leads to reduced activity in the cholesterol biosynthesis pathway that in turn results in lower levels of newly synthesized cholesterol and its intermediates,48, 49 which are essential for the synthesis of myelin.50