Ighm^tm1Msn

A secretory isoform specific mutation was achieved by deleting a 1.8 kb fragment encompassing sequence encoding a portion of the secretory tailpiece and the stop codon and polyadenylation sites. A single loxP site was left in place of the deleted sequence after the in vitro removal of the floxed tk-neo selection. Serum Igh-6 was undetectable in homozygous mutant mice. Flow cytometric analysis showed increased expression of the surface isoform. Study of heterozygous mice using allotype specific antibodies indicated that the augmented surface expression was a result of affected RNA processing rather than compensation. (J:49405)