A floxed cassette containing the rapamycin binding domain of the FK 506 binding protein fused to the C terminus of Cdh5, an IRES, a mutated form of the rapamycin binding domain of mammalian target of rapamycin (FRB*), containing an HA-tag at its C terminus, fused to the C terminus of Ptprb, and an FRT flanked hygromycin gene was inserted via recombinase mediated cassette exchange using Cdh5tm1Dvst. In lungs from homozygous mice wild-type protein is not expressed and expression of the mutant Cdh5/Fkbp protein is detected at levels similar to wild-type protein in control mice. Expression of the mutant Ptprb/Mtor* protein is about 30% of the level of expression of the endogenous protein in lungs from homozygous mice. Dissociation of heterodimers of the two mutant proteins is impaired in the presence of rapalog.