The current invention describes a nucleic acid comprising in a 5′ to 3′ direction a) a first nucleic acid encoding a heterologous polypeptide without an in frame stop codon, b) a second nucleic acid beginning with a 5′ splice donor site and terminated by a 3′ splice acceptor site comprising an in frame translational stop codon and a polyadenylation signal, and c) a nucleic acid encoding i) at least a fragment of a transmembrane domain, or ii) a signal peptide for a GPI-anchor.