The targeting construct was linearized by NotI digestion and electroporated into R1 ES cells. Cells were plated without feeder layers on gelatin-coated plates and transformed clones were selected in the presence of 200 μg/ml G418 (Gibco- BRL) and 2 μM ganciclovir (Syntex, Inc.) for eight days. Drug-resistant clones were picked, expanded on gelatin-coated plates, and genotyped by Southern blotting and PCR analysis (see below).