Alexion Pharmaceuticals, a subsidiary of AstraZeneca, is an American pharmaceutical company headquartered in Boston, Massachusetts that specializes in orphan drugs to treat rare diseases. Its products include eculizumab with $4.064 billion in 2020 revenues and ravulizumab with $1.076 billion in 2020 revenues, both used to treat the rare disorders of atypical hemolytic uremic syndrome and paroxysmal nocturnal hemoglobinuria; asfotase alfa with $731 million in 2020 revenues, used to treat hypophosphatasia; sebelipase alfa with $117 million in 2020 revenues, used to treat lysosomal acid lipase deficiency, and andexanet alfa with $78 million in 2020 revenues, used to stop life threatening or uncontrollable bleeding in people who are taking rivaroxaban or apixaban. With costs that can reach as much as $2 million per year, the drugs manufactured by Alexion are some of the most expensive drugs worldwide.