The open reading frame of the targeted gene was replaced by nuclear-localized beta-galactosidase (nLacZ; using an SV40 nuclear localization signal) using homologous recombination in a 129-derived embryonic stem (ES) cell line. A LoxP-flanked neomycin selectable marker used for ES cell selection was removed by subsequent crossing to a mouse line expressing germline Cre recombinase. (J:159435, J:189062)