SummaryBy analysing N2 mice from a cross between the inbred C57BL strain B10.Q and the NMRI-related NFR/N strain, we recently identified a quantitative trait locus (QTL) influencing litter size. This locus is now denotedFecq4, and it is present on the murine chromosome 9. In the present paper, we describe how theFecq4fragment originating form the NFR/N mouse strain will affect B10.Q mice by means of breeding capacity, super-ovulation rate and embryonic developmentin vitro. Our results show that both the breeding capacity (number of pups produced/breeding cage during a 5 months period) and the mean litter size are significantly increased inB10.Q.NFR/N-Fecq4congenic mice. Furthermore.B10.Q.NFR/N-Fecq4congenic mice (both homozygous and heterozygous) did respond much better to super-ovulation than wild-type mice, resulting in a dramatically increased yield of fertilized 1-cell embryos. In addition, embryos containing theFecq4fragment were easy to cultivatein vitro, resulting in a higher yield of embryos reaching the blastocyst stage. We propose thatB10.Q.NFR/N-Fecq4congenic mice may be used to improve breeding or super-ovulation rate in different types of genetically modified mice (on C57BL background) that exhibit severe breeding problems. TheFecq4fragment has been described in detail, and the possible role of polymorphic candidate genes near the linkage peak (58 Mb) has been discussed. Genes of the cytochrome P450 family (1, 11 and 19), such asCyp19a1, are assumed to be particularly interesting, since they are known to exhibit female-associated reproductive phenotypes, affecting the ovulation rate, if mutated.