A Frt-flanked neomycin selection cassette replaced exon 39. The pGK-neo cassette was subsequently removed by crossing the mice carrying the targeted allele to Flp-deleter mice. qPCR and in situ hybridization analyses of P1 homozygous mutant mice confirmed that expression of the CD3 isoform is abolished while expression of isoforms CD1 and CD2 is maintained. However, expression of CD2 is upregulated and CD2 is found to be ectopically expressed in support cells and Kollikers organ, rather than being restricted to IHCs and OHCs as in wild-type controls.