In utero microinjection and electroporation

In utero microinjection and electroporation was performed at E12 as described^{69 (link)}. Briefly, timed-mated CD1 mice (Charles River Laboratories) were anesthetized and the uterine horns were exposed. 1 µl of DNA solution containing 0.5 µg/µl of pCAG-mCherry and 2 µg/µl of pCAG-Let-7 sensor or pCAG-miR1-sensor in 10 mM Tris (pH 8.0) and 0.01% Fast Green was injected per embryo. Forceps-type electrodes (Nepagene) with 5-mm pads were used for electroporation (five 50-msec pulses of 25 V). Electroporated embryos were collected 18–24 hours post-electroporation and processed for immunostaining.