The Kiss1 gene encodes a neurotransmitter whose expression in the brain is essential for reproduction. Previous studies with in situ hybridization and immunocytochemistry have identified Kiss1 expression in several hypothalamic nuclei, including the arcuate. To determine whether Kiss1 expression is limited to the hypothalamus, we developed a transgenic mouse that expresses a marker gene (tdTomato) if and only if Kiss1 expression is turned on. tdTomato encodes a red marker protein that is constitutively expressed, labeling the cell forever as having once expressed Kiss1. We looked for tdTomato labeling in brain sections to identify areas that reveal expression of Kiss1 at some time during development. Surprisingly, we found tdTomato labeling outside of the hypothalamus. Possible causes include Kiss1 expression early in development or low-level Kiss1 expression in adulthood. Distinguishing between these possibilities has important implications for developing tools to genetically manipulate Kiss1 neurons in adult animals. We will test the hypothesis that Kiss1 is widely expressed at low levels in the adult brain. Kiss1 transcription will be mapped in two ways. First, brain sections will be examined for the presence of green fluorescent protein (GFP). GFP is expressed under the Kiss1 promoter, but it may not mark low levels of Kiss1 transcription. Therefore, Kiss1 expression in adulthood will also be tested by in situ hybridization. In situ hybridization radioactively marks Kiss1 mRNA, illuminating even low levels of transcription. Comparing in situ hybridization between wild-type and Kiss1 knockout mice may reveal low levels of Kiss1 mRNA outside the adult hypothalamus— and open new possibilities for a physiological role of Kiss1 signaling that extends beyond reproduction.