Halofuginone is a halogenated derivative of febrifugine, a natural quinazolinone-containing compound found in the Chinese herb D. febrifuga. It has antimalarial and anticoccidial actions. In mammals, halofuginone at 10 ng/ml down-regulates Smad3, blocking TGF-β signaling and preventing both the differentiation of fibroblasts to myofibroblasts and the transitioning of epithelial cells to mesenchymal cells. Through this action, halofuginone blocks fibrosis and tumor progression in a variety of different models. This compound also competitively inhibits prolyl-tRNA synthetase (Ki = 18.3 nM), activating the amino acid starvation response. This prevents the differentiation of TH17 cells, blunting an autoimmune response.