Recombinase mediated cassette exchange involving Gt(ROSA)26Sortm1Rkuhn introduced into the locus a CAG promoter, floxed stop cassette and a fusion of the mouse cDNA containing the amino acid substitution of glutamic acid for valine at position 673 and an N-terminus nuclear export signal to the modified estrogen receptor (ERT2). The mutation is equivalent to the human mutation V600E.