The Sanger Institute Research Support Facility (RSF) provides 50 to 100 pseudo-pregnant Fl hybrid mice a week to surgery for use as embryo recipients. This requires maintaining a pool of female Fl's from which individuals in oestrus are selected for mating with vasectomised stud males. During a standard refresh of the background colonies mating trios (F0) were crossed to provide offspring from C57BI6/J (B6) and CBA/Ca (CBA) parental colonies. Reciprocal crosses: B6 x CBA and CBA x B6 were set to produce Fl offspring. This allowed use of male and females from the two parental strains during the expansion phase for supply. Fl mice produced and their body weights are important supply criteria, only females reaching 19g by 8 weeks of age entered into the pool. To improve Fl recipient littering performance this weight criteria was increased to 20g resulting in fewer females reaching and maintaining the required weight. Observations and data collection by technicians managing the pool supported the original pre-refresh breeding strategy that the B6CBAF1 gained and maintained weight better in comparison to the CBAB6F1. As part of the continual colony review and data analysis it was decided to return to breeding Fl's exclusively to the B6CBA.
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