Abstract: The present invention provides a polypeptide having the formula: St-R1-S1-Q-S2-R2 wherein St is a stalk sequence which, when the polypeptide is expressed at the surface of a target cell, causes the R and Q epitopes to be projected from the cell surface; R1 and R2 are a Rituximab-binding epitopes each having the an amino acid sequence selected from the group consisting of SEQ ID No. 1, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 or a variant thereof which retains Rituximab-binding activity; S1 and S2 are optional spacer sequences, which may be the same or different; and Q is a QBEnd1O-binding epitope having the amino acid sequence shown as SEQ ID No. 2 or a variant thereof which QBEnd1O-binding activity. The invention also provides a nucleic acid sequence encoding such a polypeptide and uses thereof in adoptive cell transfer.