Exon 2 was disrupted by the insertion of a neomycin selection cassette. Aberrant transcripts were detected by Northern blot and RT-PCR analyses of mutant mice. The use of various cDNA fragments and sequence analysis indicated that the mutant transcripts lacked portions of exon 2 and contained neomycin sequence. Southern blot analysis confirmed that the aberrant splicing was not a result of incorrect recombination. No protein was detected by immunostaining of sperm obtained from homozygous mutant mice. (J:38658)