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rin primates and large farm species commonly have a single offspring. Previously, several mutations have been identified in sheep which increase the naturally low ovulation rate; although in some cases homozygous ewes are infertile. In the present study we present a detailed characterization of a novel mutation in growth differentiation factor 9 (GDF9), found in Icelandic Thoka sheep. This mutation is embryo survival are traits of high economic importance, mutations in oocyte-specific BMP15 (FecX) increase differentiation factor 9 (GDF9) gene is also associated on one farm could be traced back to a ewe named Thoka ovulation rate by about 1.0 in heterozygous carriers,DOI: 10.1530/REP-09-0193but result in infertile ‘streak ’ ovaries in homozygotes q 2009 Society for Reproduction and Fertility(Jonmundsson & Adalsteinsson 1985, Adalsteinsson et al. 1989). The so-called ‘Thoka ’ gene was introducedwith increased numbers of offspring born alive being advantageous for efficient livestock production with a reduced environmental footprint. As a result of studies in sheep, a species having close parallels to humans in the regulation of ovarian function throughout life, majo