Tamoxifen was prepared by first dissolving in ethanol (20 mg/500 μl) and mixing this solution with 980 μl corn oil for a final concentration of 20 mg/ml. Ethanol was then removed with a heated speed vacuum. Mice containing CreERT2 that were ~2 months old were injected with approximately 200 μl tamoxifen solution (200 mg/kg) once a day over 5 days. Animals were monitored for adverse effects, and if these became apparent, treatment was stopped. One week after treatment ended, animals were processed as described below for ISH or localization of XFP.