Combining the pooled drug arms of the study, investors polled by Schoenebaum were looking for a 6.1% improvement in lung function, adjusted for placebo. Vertex did not provide the actual result Thursday. Vertex stands to grow its nascent cystic fibrosis business substantially if a VX-809-Kalydeco combination therapy is ultimately approved since about half of the 70,000 cystic fibrosis patients worldwide carry two copies of the F508del gene. Kalydeco was approved in January as a single-drug therapy for about 4% of cystic fibrosis patients carrying the G551D genetic mutation. First-quarter Kalydeco sales totaled $18.4 million. Cystic fibrosis is caused by genetic mutations that result in a malfunctioning or missing protein known as CFTR required for the regulation of sweat production, mucus and certain aspects of digestion. Defective or missing CFTR proteins in lung cells results in the formation of thick, sticky mucus that leads to restricted airflow, chronic infections and lung damage. Patients with the most common F508del mutation have missing or insufficient CFTR protein on the surface of cells. VX-809 is designed as a "corrector" drug that increases the amount of CFTR protein on the cell surface. Kalydeco, on the other hand, is a "potentiator" designed to improve the function of the damaged CFTR proteins. Kalydeco was approved in January for cystic fibrosis patients with the G551D mutation. These patients have sufficient CFTR proteins on the surface of cells but the proteins are damaged and don't work correctly. The phase II combination study treated cystic fibrosis patients with the F508del mutation with four weeks of VX-809 alone to increase the amount of CFTR protein, followed by another four weeks of VX-809 combined with Kalydeco to improve the function of the new CFTR protein. Three doses of VX-809 were studied along with a higher dose of Kalydeco. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: firstname.lastname@example.org. Follow TheStreet on Twitter and become a fan on Facebook.