We then compared Prep-/- mice with Prep+/- mice under WD/CCl4 treatment and found that liver fibrosis, as assessed by collagen gene expression, Sirius red staining, and α-smooth muscle actin (α-SMA) protein level, was significantly increased in the Prep-/- mice (Fig. 1d–g), indicating increased liver fibrosis after Prep knockout. The gene discussed is PREP; the disease is Wilson disease.