The first snoRNA 50 (mU50) sequence was replaced with a 68 bp multiple cloning site from pBluescriptII KSII plasmid containing an EcoRV site. The second mU50 site was replaced with a floxed neomycin resistance cassette. Cre-mediated recombination removed the resistance cassette. Northern blot analysis confirmed the absence of expression in the spleen. (J:206419)