PDE4A and Huntington disease: The striatal mRNA expression of PDE4A in R6/2 mice (ie, one of the first transgenic HD models created by inserting a copy of the five untranslated regions, exon 1 with about 150 CAG repeats, and part of intron 1 of the human HD gene into the mouse genome) has been found to be proportionately decreased as the animals age.29