A cDNA encoding a MerCreMer fusion protein and an FRT-flanked neomycin cassette were recombined in-frame with the Ly6a ATG start codon (exon 2) via homologous recombination in embryonic stem (ES) cells. The MerCreMer double fusion protein incorporates a cre recombinase cDNA sequence flanked by mutated murine estrogen receptor (mer) ligand binding domains (amino acids 281-599, G525R). Resultant mice were crossed with Flpe mice to delete the neomycin cassette. Expression of Ly6a is eliminated in the targeted allele.