A 3.5 Kb fragment from a plasmid carrying a rat insulin Ins2-tTA cassette and a 3.9 Kb fragment from a plasmid carrying TetO7 cloned upstream of Cre-encoding DNA were co-injected into fertilized oocytes to produce double-transgenic mice. Five lines were generated and one line showed cre activity doxycycline dependant manner, on with the absence and off with the presence of doxycycline, in islet cells. (J:244123)