Protein structure and normal expression pattern of MUC1. Schematic diagram of the size and structure of the full-length MUC1 protein relative to the plasma membrane. The horizontal bar indicates the distance that most cell surface proteins extend into the pericellular space. The three major domains of MUC1 are indicated to the left: (1) an N-terminal, extracellular domain (ECTO); (2) a single, membrane spanning domain (TM); and (3) a C-terminal cytoplasmic domain (CT). A list of epithelial and non-epithelial human tissues that normally express MUC1 is indicated to the right.