This transgene directs the expression of Cre recombinase under the control of a phosphoglycerate kinase promoter, with integration site effects. Expression of this transgene is under dominant maternal control, probably due to cis acting effects at the integration site. SNP typing indicates that it is likely the transgene integrated into a Chr 4 locus of BALB/c origin. Activity is thought to commence at the diploid phase of oogenesis, and directs complete recombination in all tissues of progeny derived from mothers carrying the transgene.