SLC38A4 (ATA3), associated with the plasma membrane, mediates the sodium-independent uptake of arginine and lysine. SLC38A4 was first identified on the basis of its similarity to SLC38A1 and SLC38A2. Like those two transporters, it can mediate the sodium-dependent uptake of neutral amino acids in cultured cells transfected with an expression vector, but it does so very inefficiently and its role, if any, in neutral amino acid uptake in vivo is unclear. By Northern blotting, SLC38A4 is abundant in liver and undetectable in all other tissues tested, including heart, placenta, kidney, and intestine (Hatanaka et al. 2001).