The 5' LTR of murine endogenous retrovirus with leucine tRNA primer (MuERV-L; MERVL; Erv4) and the primer binding site drive expression of tamoxifen-inducible Cre recombinase (2C::ERT2-Cre-ERT2). Two flanking ERT2 segments were inserted to make Cre activation less leaky. The transgenic vector was injected into CB6 (BALB/c and C57BL/6) embryos and the resultant strain was subsequently maintained on the same mixed genetic background by the donating laboratory. This is founder line 657.