The present invention provides a chimeric antigen receptor (CAR) signalling system comprising； (i) a receptor component comprising an antigen binding domain, a transmembrane domain and a first binding domain； and (ii) an intracellular signalling component comprising a signalling domain and a second binding domain which specifically binds the first binding domain of the receptor component； wherein, binding of the first and second binding domains is disrupted by the presence of an agent, such that in the absence of the agent the receptor component and the signalling component heterodimerize and binding of the antigen binding domain to antigen results in signalling through the signalling domain, whereas in the presence of the agent the receptor component and the signalling component do not heterodimerize and binding of the antigen binding domain to antigen does not result in signalling through the signalling domain.
