Transthyretin (TTR) is a thyroid hormone transport protein that is secreted by hepatocytes into the serum and by the choroid plexus epithelium into the cerebral spinal fluid. The protein is not made elsewhere in adult animals in significant amounts. We find that the start site for mRNA synthesis is the same in both cell types. The sequences required for mouse TTR expression in cultured hepatocytes include an enhancer at -1.86 to -1.96 kbp and a promoter-proximal region at -70 to -200 bp relative to the mRNA cap site. We demonstrate that in transgenic mice these regulatory regions (approximately 300 bp) are sufficient for quantitatively normal expression of a TTR minigene in hepatocytes, but not for restricted expression in the choroid plexus cells of the brain. Instead, they direct aberrant widespread expression in regions of the brain outside the choroid plexus. With 3 kbp of upstream sequence the TTR minigene is expressed specifically in the choroid plexus as well as in the liver, demonstrating the normal cell type specificity for TTR. These results suggest that 3 kbp of upstream sequence contains positive element(s) required for choroid plexus expression which are distinct from those utilized in the hepatocyte, and may also contain negative element(s) that function to suppress transcription in other brain cell types.