A 10-kb region spanning exon 2 through the stop codon in exon 7 was replaced by a neomycin selection cassette. The deleted region encoded the nuclear localization signals, serine patches, and a conserved carboxyl terminus. Northern blot analysis using probes corresponding to regions outside of the deleted interval showed the presence of a hybrid mRNA. Sequence analysis of the aberrant transcript showed a cryptic splice junction between exon 1 and the inserted neo gene. Western blot analysis using an antibody raised against the carboxyl terminus showed a lack of normal protein, but could not confirm the absence of an 82 residue peptide putatively translated from the exon 1 fragment. Attempts to generate an antibody for the largely hydrophobic amino terminus were not successful.