The HPRT minigene is a selection cassette used for gene targeting in mouse embryonic stem (ES) cells and, it is unique since selection may be applied for its presence and absence. This minigene has two exon clusters separated by a small intron and splicing sequences. We find these exon clusters splice into exons from the target gene forming two different classes of chimeric transcripts. The first class is expressed by the endogenous promoter and includes upstream target gene exons spliced into minigene exons 3-8. The second class is expressed by the minigene's PGK promoter and includes minigene exons 1-2 spliced into downstream target gene exons. These chimeric transcripts may produce chimeric proteins that could influence phenotype. Therefore, we have designed two floxed HPRT minigenes that permit removal of either the 5' half of the minigene or the entire minigene via Cre-mediated recombination.