Vertex got its start ­making a hepatitis C drug, but it has more recently homed in on medicine that treats symptoms of cystic fibrosis, a genetic disease that afflicts 75,000 people worldwide. Its two bestselling drugs, Orkambi and Kalydeco, account for 87% of its $2.6 billion in revenue. And in February, Vertex received approval of a third CF drug, Symdeko, which earned $186 million in its first quarter on the market—more than twice what analysts expected. The success of the drugs has helped Vertex more than double its revenue in three years.