A LacZ-PGK-neo cassette was inserted into exon 1 via homologous recombination. Northern analysis using the 3' end as a probe detected a faint band that when analyzed was found to be a nonfunctional transcript. This aberrant transcript contains a long stretch of antisense PGK, part of intron 2 and downstream exons. Beta-galactosidase was expressed under the control of the endogenous promoter.