They were also validated with the Hoxa5 conditional allele. The efficiency of these treatments was analyzed at P7 using a transgenic reporter line. For postnatal treatment, different dosages of tamoxifen were administered by intragastric injection to the pups during 3 or 4 days after birth. To circumvent this problem, pups were collected from tamoxifen-treated pregnant dams by caesarian section at E18.5 and given to foster mothers. However, delayed delivery and/or mortality of pups due to difficult delivery were always observed. For the late gestation treatment with tamoxifen, different protocols of complementary administration of progesterone and estrogen were tested. The efficiency of recombination was evaluated in the brain, which is known to be more problematic to target. In this context, using the ubiquitous CMV-CreER T2 transgenic mice, we have tested and validated two procedures to achieve recombination just before and just after birth. Although powerful protocols of induction have been developed for gene inactivation at adult stages or during embryonic development, induction of recombination at late gestational or early postnatal stages is still difficult to achieve. Temporal control of site-specific recombination is commonly achieved by using a tamoxifen-inducible form of Cre or Flp recombinases.

Red swan cre