Cre recombinase-induced recombination between a loxP site at the App locus (Apptm1Yah) and a loxP site in Runx1 (Runx1tm1Yg) on the oposite Chr 16 homolog (in trans) resulted in duplication of the intervening chromosomal segment. To achieve this, a male mouse carrying Runx1tm1Yg was mated to a female bearing Hprttm1(CAG-cre)Mnn, and a female offspring with both targeted alleles was bred with a male carrying Apptm1Yah. A daughter carrying all three modifications, in whose oocytes recombination occurred, was mated to C57BL/6 males to obtain offspring with the duplication. (J:82809)