Mark Cuban is expanding Cost Plus Drugs into custom-made treatments for rare diseases. The billionaire has spent the last five years working to lower the cost of generic medicines, including chemotherapy and antifungal ointments, for American patients. But a different type of medicine began arriving at his Dallas factory earlier this year.

One shipment contained DNA-like molecules created by California chemists to correct a rare genetic mutation in a single patient. Another carried similar molecules developed with Dutch researchers to halt a condition that causes adults to gradually lose muscle control. These deliveries mark the first test of Cuban’s new vision for the company: disrupting the business of bespoke rare disease therapies.

“It was an obvious chance for us to help those in that unfortunate situation,” Cuban wrote in an email.