Thyroid transcription factor-1 (TTF-1), a member of the Nkx2 family of homeodomain-containing proteins, is involved in binding to and in activating the promoters of several important genes in the thyroid, lungs, and brain, and in regulating expression of these tissue-specific genes. We investigated potential roles of sheep (Ovis aries) TTF-1 in regulating cell fate and organ morphogenesis and in controlling puberty and reproductive capability of females. We amplified and cloned the sheep TTF-1 full-length DNA for the first time, analyzed its functional domains and regions, predicted molecular structure of its homeodomain and DNA-binding sites, and examined its expression in pituitary, brain, thyroid gland, ovary, and hypothalamus. We found that sheep TTF-1 has a high degree of homologous identity with that of other mammals, and it has several important domains including domain N, DNA-binding domain, domain C, TN-domain, domain I, and NK2-SD. The DNA-binding domain of sheep TTF-1 has 10 potential DNA-binding sites and is a novel mammalian homeodomain that shows considerable sequence homology with the corresponding rat homeodomain. Several functional regions in sheep TTF-1 share high sequence identity with rat TTF-1, indicating that these regions may have the same activity as in the rat. Expression of TTF-1 in several specific tissues implies that sheep TTF-1 in involved in sheep sexual development and reproductive capability. These results suggest a role of sheep TTF-1 in enhancing sheep reproduction performance and we propose it as a candidate gene for selection.