Phenylalanine hydroxylase-stimulating protein, also known as pterin-4 alpha-carbinolamine dehydratase (PHS/PCD), was purified from rat and, for the first time, from human liver. We obtained their complete protein primary sequence using a combination of liquid secondary ionization mass spectrometry/tandem quadrupole mass spectrometry, electrospray ionization mass spectrometry, and Edman microsequence analysis. The amino acid sequences of human and rat PHS/PCD were found to be identical. Surprisingly, the primary structure of PHS/PCD is also essentially identical to a protein of the cell nucleus, named dimerization cofactor of hepatocyte nuclear factor 1 alpha, recently reported to be involved in transcription (Mendel, D. M., Khavari, P. A., Conley, P. B., Graves, M. K., Hansen, L. P., Admon, A., and Crabtree, G. R. (1991) Science 254, 1762-1767).