An frt-flank neo cassette with a 3' lox P site was inserted upstream of the open reading frame and an additional lox P site was inserted downstream of the open reading frame. Flp-mediated recombination was used to remove the neo cassette and germ line cre-mediated recombination was used to remove the open reading frame. The absence of protein product was confirmed by the absence of protein product in the plasma. (J:129742)