This present invention provides an expression vector system that uses alternative RNA processing to express in a single cell a polypeptide in both membrane-bound and soluble forms. By incorporating a mimetic structure of the 3′ terminal region of human mu gene and introducing other exogenous genetic elements, an artificial gene can be constructed that is capable of simultaneously expressing membrane-bound and secreted forms of polypeptides in myeloma cells and other cells of the B lymphocyte lineage, as well as in non-B cells. If an immunoglobulin heavy chain is co-expressed with a light chain using this vector, whole antibodies can be produced that are both displayed on the surface of a single cell and secreted into the cell culture supernatant. Membrane-bound antibodies facilitate isolation and expansion of those cells displaying antibodies with desired antigen binding characteristics, while secreted antibodies facilitate identification of antibodies having desired biological function(s).