Solute carrier (SLC) proteins play critical roles in ion and nutrient transport and serve to maintain solute homeostasis. At present, over 350 human SLC genes are known and have been assigned to 47 SLC gene families. We have recently performed a large-scale gene discovery screen to define the complete set of SLC genes in four species - human, mouse, Xenopus laevis and Xenopus tropicalis - and identified on the basis of molecular phyologenetic analysis three novel vertebrate SLC38 gene family members, namely SLC38A7, SLC38A8, and SLC38A10. The SLC38 family comprises sodium-coupled neutral amino acid transporters and was long thought to contain only 6 members. We performed by real-time PCR a detailed gene expression analysis covering a range of 21 adult murine tissues. We found that Slc38a7 and Slc38a10 were ubiquitously expressed in all organ tested with highest levels of expression in vesicular glands and testes, respectively. In contrast, Slc38a8 was exclusively expressed in the eye. It was recently shown that members of the Slc38 family –in particular Slc38a3 and Slc38a5- are the major mediators of glutamine transport in retinal Müller cells. Glutamine transport is crucial for the glutamate-glutamine cycle which occurs between neurons and glia. Given the restricted expression pattern of Slc38a8 in the eye, we aimed to determine if this orphan member could play a role in the glutamine-glutamate cycle. For that purpose, we cloned Slc38a8 into an expression vector and tested its transport properties by heterologous expression in Xenopus oocytes. In a set of preliminary studies however, we failed to observe glutamine or glutamate uptake. We are currently determining the cellular localization of the three Slc38 transporters by immunohistochemistry and in situ hybridization and further investigating their substrate specificity.