Tezacaftor (VX-661) is primarily used in the treatment of cystic fibrosis, a genetic disorder that affects the lungs and digestive system. It works by improving the function of the defective CFTR protein, which is responsible for the movement of salt and water in and out of cells. By enhancing this protein's activity, Tezacaftor helps to reduce the buildup of thick, sticky mucus in the lungs, thereby improving breathing and reducing the risk of infections. It is often used in combination with other medications, such as ivacaftor, to achieve better therapeutic outcomes. This combination therapy has shown significant improvements in lung function and quality of life for patients with specific mutations of the CFTR gene.