4-hydroxytamoxifen (Sigma, H7904) was dissolved in ethanol, emulsified in sunflower oil (Sigma Catalog, S5007) and then sonicated for 10 minutes at a concentration of 10 mg/ml. For pregnant mothers, 4OH-tamoxifen was used at a final dose of 3 mg per female (mother weighed 30 gms) and intraperitoneal injections were performed 24 hours prior to sacrificing the animals. Embryos were harvested from mothers, and processed for X-gal staining. For all other injections, tamoxifen (Sigma, T5648) was dissolved in ethanol and emulsified in sunflower oil at a concentration of 30 mg/ml. All animals were injected intraperitoneally with 3 mg of tamoxifen per 10 gm of animal body weight. Mice aged 6-months or older were injected with lower doses of 1.5 mg of tamoxifen per 10 gm of animal body weight for three consecutive days.