Gene silencing has been effective in mouse models of HD that are heterozygous for the mHTT gene.The results are promising as_______ .

A)the injection into both sides of the mouse brain resulted in reduced mHTT
B)the study demonstrated the ZFN only targeted the mHTT allele and left the normal HTT expression level unchanged
C)the injection into one side of the mouse brain affected both sides of the brain
D)both mHTT and HTT were down regulated