The floxed region containing exon 4 was excised from Mtmr2tm1Abol by crossing with CMV-Cre transgenic mice, causing a frameshift from either ATG start site of translation (in exon 1 or exon 3). No mRNAs containing exon 4 or 3' exons could be detected by RT-PCR from tail, brain, muscle, and sciatic nerve from mutants. Western blot analysis showed that protein was absent from brain lysates from mutant animals.