The RP11-134D15 bacterial artificial chromosome (BAC) was obtained and retrofitted to contain a human HPRT complementary sequence (containing exon 1, intron 1, exon 2, and part of intron 2), a mouse 3' Hprt homology arm, an I-Sce linearization cut site, and a mouse 5' Hprt homology arm; all inserted into the sacB gene in the BAC vector backbone. Next, a beta-galactosidase (lacZ) and frt-flanked kanamycin cassette were inserted directly at the ATG of the human nuclear receptor subfamily 2, group F, member 2 (NR2F2) gene; this created the Ple143-lacZ BAC (bEMS85, NR2F2). The kanamycin cassette was transiently removed via FLP recombination; leaving the lacZ gene followed by a single frt site. The final modified BAC was used as the source for the Ple143-lacZ transgene. The Ple143-lacZ transgene was targeted as a single copy knock-in to the Hprtb-m3 mutant locus on the X Chromosome via electroporation into mEMS1202 embryonic stem cells. (J:164356)