An IRES element, a cre/ESRI fusion gene. and a PGK-neo selection cassette were inserted into exon 16 3' to the stop codon. A cryptic polyA site 3' of the stop codon was deleted in order to permit cre expression. Cre expression was confirmed by EGFP expression after tamoxifen treatment of mice also carrying the Tg(CAG-Bgeo/GFP)21Lbe transgene. MCM2 expression was reduced to 62% of normal in heterozygotes and 35% in homozygotes.