Shares of biotech Scholar Rock more than quadrupled on Monday, adding some $1.9 billion to its market value, after the company said one of its drugs improved motor function in children with a genetic muscle-wasting disease. Scholar Rock said children with spinal muscular atrophy who had significant improvements on measures such as standing and walking after taking the company’s drug, whose scientific name is apitegromab, in a final late-stage study. The company plans to seek U.S. and European Union approval for the medicine in the first quarter of next year.