Protein actively involved in the transport process MBTR is a transmembrane regulator of cystic fibrosis☝🏻 This protein is involved in the transport of chlorine ions across the cell membrane. Intracellularly, it is located on the surface of the apical membrane. MBTR is localized mainly in the epithelial cells of the respiratory tract, salivary and sweat glands, pancreas, and intestine.