This study looks at how a new medicine, VX-121/tezacaftor/deutivacaftor (VX-121/TEZ/D-IVA), works in kids with Cystic Fibrosis (CF), a condition that affects the lungs. The study focuses on kids who have a certain change, called a triple combination responsive (TCR) mutation, in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. This gene helps control how water and salt move in and out of cells.

The study will check how the medicine moves through the body (pharmacokinetics), if it is safe and easy to take (safety and tolerability), and if it helps improve health (efficacy). Participants must have stable CF and at least one TCR mutation. They cannot join if they have had an organ transplant, cancer, serious liver problems, or certain lung infections.

• The study is for children aged 1-11 years.
• Participants should not have serious liver issues or certain lung infections.
• Other criteria may apply, and participants will be informed about them.