A targeting vector was designed with a loxP-flanked STOP cassette (PGK-Neo-polyA followed by transcriptional stop sequence) just upstream of a green fluorescent protein/beta-galactosidase fusion protein sequence with SV40 nuclear localization signal (GFP-NLS-lacZ or GNZ). This entire construct was inserted into the Gt(ROSA)26Sor locus via electroporation. (J:135149)