A PAC (RPCI21) containing a 125 kb 5'upstream region and 31.2 kb 3'downstream region of the mouse Aqp2 gene was modified by ET-recombination so that a cassette encoding a codon-improved cre recombinase sequence was inserted at the ATG of Aqp2. This modified fragment was injected into FVB/N oocyte pronuclei. A founder line with 2 copies of the transgene was established. (J:148058)