Expression of bacteriophage P1 Cre recombinase, bearing the nuclear localization signal peptide of SV40 large T antigen, is directed by the promoter of the mouse gene encoding calcium/calmodulin-dependent protein kinase II alpha. Expression of the transgene was demonstrated by RT-PCR analysis, immunoblot analysis and a reporter assay for Cre recombinase activity. Activity is present specifically in the nuclear fraction of the cerebrum, although a low level (1-10% of that in the cerebrum) of "leaky" expression is observed in the cerebellum as well.