Achillion Pharmaceuticals, Inc. is a biopharmaceutical company. The Company is engaged in the discovery and development of small molecule drug therapies for infectious diseases and immune system disorders. The Company is focused on its complement inhibitor platform, directed at advancing small molecule compounds that have the potential to be used in the treatment of immune-related diseases associated with the alternative pathway of the complement system. The complement system is part of the human innate immune system. Its clinical compound from its complement inhibitor platform is ACH-4471. ACH-4471 is a potent and specific inhibitor of factor D, which demonstrated complete inhibition of the complement alternative pathway in human healthy volunteers in a Phase I clinical trial. The Company has discovered and advanced multiple compounds into clinical development, including sovaprevir, odalasvir, and ACH-3422 in its hepatitis C virus (HCV) program.