Nolan and Jack Willis, twins from upstate New York, and just 10 other boys took part in a clinical trial that led to the approval last fall of the very first drug to treat their rare, deadly muscle disease.

Now the Willis boys are again test cases as a different type of medical question comes to the fore: whether insurers will cover the controversial drug, Exondys 51, which can cost more than $1 million a year even though it’s still unclear if it works.

The boys’ insurer, Excellus BlueCross BlueShield, refused to cover the cost of the drug because the twins, who are 15, can no longer walk. Their disease, Duchenne muscular dystrophy, overwhelmingly affects boys and causes muscles to deteriorate, killing many of them by the end of their 20s.

“I’m cycling between rage and just sadness,” their mother, Alison Willis Hoke, said recently, on the day she learned that an appeal for coverage had been denied. For now, the company that sells the drug, Sarepta Therapeutics, is covering the treatment’s costs, but Mrs. Hoke does not know how long that will last.