A Frt-flanked neomycin selection cassette replaced exon 35. 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 CD1 isoform is abolished while expression of isoforms CD2 and CD3 is maintained. However, expression of CD2 is downregulated relative to that in wild-type controls.