Sortase molecules and methods described herein allow for the construction of a CAR or CAR member, e.g., in situ, on a CARX, e.g., CART, cell. For example, sortase mediated transfer of an antigen binding domain, e.g., a scFv, onto a CAR member having a sortase acceptor motif in place of an antigen binding domain can provide for a complete CAR member on a cell wherein the cell does not comprise nucleic acid that encodes the complete CAR member.