The present invention provides a nucleic acid construct comprising the following structure: A-X-B in which X is a nucleic acid sequence which encodes a cleavage site; and A and B are nucleic acid sequences encoding a first and a second chimeric antigen receptor (CAR), each CAR comprising: (i) an antigen-binding domain; (ii) a spacer (iii) a trans-membrane domain; and (iv) an endodomain wherein the antigen binding domains of the first and second CARs bind to different antigens; wherein one of the first or second CARs is an activating CAR comprising an activating endodomain and the other CAR is an inhibitory CAR comprising a ligation-off inhibitory endodomain; and wherein: (a) the first and/or second CAR comprises an intracellular retention signal; and/or (b) the signal peptide of the first or second CAR comprises one or more mutation(s) such that it has fewer hydrophobic amino acids.