Doctors who gave children with cystic fibrosis a replacement copy of a defective gene say it appeared to slow the expected decline of some patients’ lungs, but called the results “modest” and say there must be major improvements before offering the treatment more widely. Cystic fibrosis is an inherited condition that fills the lungs with mucus, making people susceptible to infections that can eventually destroy the lungs; the average life expectancy is about 37 years.