Diagram of the epitope tag-modified Hdh exon 1. The wild-type Hdh exon 1 contains a short polyQ stretch (7Q) and an adjacent proline-rich region (mouse PRR, gray). An HA N-terminal epitope tag (green) was inserted between amino acids 1 and 2 of the Hdh exon 1 containing the wild-type mouse polyQ stretch ( Hdh HA7Q), while a 3xFlag N-terminal epitope tag (yellow) was inserted in the wild-type Hdh exon 1 ( Hdh 3xFlag7Q), a chimeric mouse/human exon 1 containing a normal human polyQ stretch ( Hdh 3xFlag20Q), and a chimeric mouse/human exon 1 with an expanded polyQ stretch (Hdh3xFlag140Q). The human portion of the chimeric exon 1 is indicated in light blue and darker blue (the human PRR), while the polyQ stretches are displayed in shades of red. The sequences of the epitope tags are indicated along with key restriction sites used in the construction of the gene targeting vectors. A, AlwN I; X, Xmn I; K, Kpn. I.