Primary MEFs were generated as described in a previous study . Briefly, the uterine horn was collected from adult female mice at day E12.5 and transferred to a 10 cm tissue culture dish containing cold PBS. Two forceps were used to tear the yolk sacs to isolate each embryo. Forceps were cleaned thoroughly with 70% ethanol between each embryo isolation. The embryos were culled, and a tissue piece was collected in a PCR tube for genotyping. The red tissue of the embryo was removed, and the remainder was minced with a scalpel blade and incubated with 7.5 ml trypsin-EDTA solution for 10 minutes in a 37°C, 5% CO2 tissue culture incubator. The dish was removed from the incubator and checked under a light microscope for single cells. 7.5 ml complete media was added to the trypsinised cells and the cell suspension was transferred to a 15 ml Falcon tube, and centrifuged at 1200 rpm for 5 minutes at room temperature. The trypsin was aspirated, the cell pellet was resuspended in 5 ml fresh complete media, and the cell suspension was plated in a 60 mm tissue culture dish and incubated in a 37°C, 5% CO2 tissue culture incubator. The MEFs at this stage were considered as passage 0 and were passaged and expanded for experimental use once the genotype was confirmed by allelic sequencing and immunoblotting. MEFs were cultured in Dulbecco's modified Eagle medium (DMEM) containing 10% (v/v) fetal bovine serum (FBS), 2 mM L-glutamine, 100 U/ml penicillin, and 100 µg/ml streptomycin supplemented with 1X nonessential amino acids and 1 mM sodium pyruvate.