LoxP sites were inserted to flank exons 3-5. An FRT-flanked promoterless Beta-galactosidase-neomycin cassette was inserted into intron 5. Crossing to EIIa-Cre mice excised the floxed region containing exons 3-5 and left the FRT-flanked Beta-geo. Immunohistochemical analysis of mutants confirmed absence of endogenous protein. Western blot of mutant neonate intenstinal tissue also demonstrated absence of protein. (J:92210)