The site-specific DNA recombinase Cre is being used to develop a new generation of tools for controlling gene expression in mice . Cre mediates the recombination of two directly repeated target (loxP) sites to a single loxP site, with concomitant excision of the DNA segment flanked by the loxP sites (the 'floxed' DNA). Such recombination can function to activate a gene by excising a floxed DNA segment that blocks expression because it either separates the regulatory and coding sequences of the gene  or interrupts the gene's open reading frame. Conversely, DNA excision can inactivate a gene if an essential fragment of the gene is floxed . Gene activation or inactivation in vivo can be achieved by mating two different animals, one carrying a 'target gene' with appropriately placed loxP sites and one carrying a cre transgene. In most cases, the specificity of the system is dependent upon stringent regulation of cre expression. We describe here a mouse line in which cre expression is controlled by regulatory sequences from the mouse zona pellucida 3 (Zp3) gene, which is normally expressed exclusively in the growing oocyte prior to the completion of the first meiotic division . We show that in target-bearing Zp3-cre mice, Cre-mediated recombination of the target gene apparently occurs in 100 % of oocytes. Moreover, Cre activity is not detected in the somatic tissues of most target-bearing Zp3-cre mice. Potential uses for this mouse line are discussed.