The Cre recombinase coding sequence, flanked by a nuclear localization signal and an SV40 polyadenylation site, was inserted just after the translation start site of the neurogenin 3 gene present in mouse BAC RP23-121F10, which also includes approximately 81 kb of 5' and 102 kb of 3' flanking DNA. Immunohistochemical analysis shows the same expression pattern for cre as for neurogenin 3 in adult and embryonic mice bearing this transgene. Cre recombinase activity is seen in cell types of pancreas, stomach, small intestine and colon that express or are descended from cells that express neurogenin 3 during development.